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The NETCELL simulation package - Department of : Civil and

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1. Page Setup Print Curve 3P Print Table Quit aeQ Figure 16 The File Menu Export Table This command allows the user to save the data shown in the Table window as a tab delimited text file This format is compatible with most spreadsheets and data analysis programs 35 Page Setup This command allows the user to set various printing parameters Print Curves This command prints the vehicle arrival and departure curves for the selected arcs The graph is scaled to fill a page Print Table This command prints the data in the Table window 10 2 2 Edit menu The edit menu is the standard edit menu for the macintosh or for windows The only window in which edit commands are functional is the arc selection window In that window the value in the data entry field for the arc number may be copied cut or pasted None of the other windows have any editable data in them to which the commands of the edit menu can be applied ptions lU Can t Undo 382 Cut 36H Copy C Paste U Clear Select All fA Figure 17 The Edit Menu 10 2 3 Options menu The two options in the Options menu apply to the network window only They allow the user to change the scale with which the network is drawn The Zoom In command enlarges the network view by a factor of two while the Zoom Out command shrinks the network view by a factor of two Either command can be repeated to enlarge or reduce the view further When
2. The user has the option to specify an alternate wave speed which would define a trapeziodal flow vs density diagram see Figure 6 Figure 1 represents a default case although we will see later that a more general flow density relationship such as those shown in Figures 2 and 6 can also be specified Under the cell representation of a network a three legged cell represents either a merge or diverge junction Two priority coefficients are defined for each merge they indicate the fraction of vehicles that enter the node from each approach when queues exist on both approaches We also assume that two destination specific route choice constants usually 0 or 1 are defined for each exit to a diverge they indicate the fraction of vehicles with destination d that take the corresponding exit Although the priority and route choice coefficients can vary with time in the current version of the NETCELL program they are assumed to be time invariant This restriction may be relaxed some time in the future In the cell transmission model the conventional network representation just described needs to be altered Each arc should be partitioned into sections called cells which should be traversed in one simulation clock step under free flow conditions 1 Cells are then viewed as additional network nodes themselves connected by additional arcs To avoid any confusion the arcs between cells are called links In the cell representation the
3. OD table for time 30 ODTIME 60 ODROW 0 4 4 ENDODTABLES Incident generation 6 columns 1 2 arc location starting ending jam number distance time time capacity density INCIDENT 1 0 375 350 650 0 20 ENDINCIDENTS ENDINPUT Merge priority coefficients for arcs pointing to a merge ACL 2 Aare number priority coefficient Data range 0 1 ENDROUTING 44 2 columns
4. a cell specific value indicating the number of vehicles entering 2 cell i in the time interval t t This quantity is also defined for links Links connectors between cells Maximum occupancy N t a cell specific and time specific constant representing the maximum number of vehicles that can be held in cell i at time t it is the product of the jam density and the cell length v Maximum throughput Q t a cell specific and time specific constant representing the maximum number of vehicles that can flow in or out of cell i in one clock step it is the product of the maximum flow of the arc in which the cell resides and the clock step 6 Merge priority coefficient p t and pa t input parameters for arc k and its companion arc ck at a merge junction specifying the fractions of vehicles merging from each approach when the supply of vehicles from both approaches exceeds what can be accommodated p t pex t 1 Origin zone a single node or a single cell in cell representation where traffic demands are generated and released into the network Outbound cell flow y t a cell specific value indicating the number of vehicles leaving cell i in the time interval t t 8 Packet njq t a cell specific value indicating the real valued number of vehicles in cell i at time t that are headed for the same destination d and have entered the cell in the same time interval t t 6 for t lt t Route choice c
5. cancel button 33 closes the arc selection window without updating the arc selection list The selection list will revert back to the list in effect after the last apply or done command Curve Window 125 250 375 500 625 750 875 1000 1125 1250 T M E Figure 14 The Curve Window 10 1 3 The curve window The curve window draws the inflow and outflow flow time curves for the combined selected arcs This shows the combined flow counts versus time for all the arcs selected Figure 7 shows a flow time curve for the example input file and section 5 2 discusses the relationship between the flow time curve and the arc travel time The window is resizeable and printable 10 1 4 The table window The table window shows the detailed data for the combined selected arcs For each time slice of the simulation the table shows the inflows and outflows for the arcs for that time slice and the cumulative inflows and outflows The values shown are the sum of the inflows and outflows for the arcs in the arc selection list This window is also resizeable and printable The data in it can also be exported in a format compatible with most spreadsheets to allow further data analysis 34 Sass Simulation Results 1 Tot out Out dA Figure 15 The Table Window 10 2 NETVIEW menus 10 2 1 File menu The file menu contains the commands for open and closing files printing various output windows exporting data and quitting Export Table
6. If the selected file is not a valid input file the program will display an error alert If this happens most functions within NETVIEW will be unavailable and the display windows will be empty Select open from the file menu and reselect a valid NETCELL arc cumulative flow file After the arc flow file has been read the user can generate flow time curves for the arcs in the network print graphs or tables and export the data to other files To work with another input file simply select open from the file and select a different arc flow file Only one file can be open at a time so opening a second file will close the first automatically 10 1 NETVIEW display windows The NETVIEW program presents the user with four display windows and four menus to manipulate them The windows are the network window which displays a graphical representation of the network the arc selection window which allows the user to select and deselect the arcs which are used to calculate results the curve window which displays the flow time curves for the selected arcs and the table window which displays the cumulative counts and other information for the selected arcs 10 1 1 The network window The network window shows a graphical representation of the input network Nodes are shown as small circles with connecting arcs drawn with black lines Arcs are labeled approximately in the center of the arc with the arc number The graphical layout depends on the
7. arcs and zero one or two outgoing arcs The number of incoming arcs is used to determine the type of the first cell in the arc No incoming arcs indicates that the upstream cell is an origin cell one incoming arc Flow Recieving Flow Sending Flow Density ko k kjk Figure 2 Generalized flow density relationship indicates that the first cell is an ordinary cell while two incoming arcs indicate the cell is a merge cell Similarly two outgoing arcs indicate that the last cell in the arc is a diverge cell one outgoing arc indicates an ordinary cell and zero outgoing arcs indicates a connection to a destination cell This allows us to treat all the cell types properly without actually storing the type as part of a cell An arc also stores information common to all its cells e g the free flow speed capacity jam density which are by default the same for all cells in the arc Thus 6 if the user wishes to model an uninterrupted physical facility where some of the conditions change e g due to a lane drop or change in grade s he may need to split the original arc into two or more arcs Type 0 cell Type 1 cell Type 2 cell Ordinary cell Diverge cell Merge cell Figure 3 Cell type specifications Some of the above mentioned parameters may vary with time e g If a capacity reducing incident takes place somewhere in an arc Because this is of some interest this implementation of NETCELL allows variable capacities to b
8. a list of free cohorts and free packets When a cohort is deleted it is added to the free cohort list and its packets are added to the free packet list When a cohort is created the program uses a cohort from the free list If either the free cohort list or the free packet list is empty a block of additional cohorts or packets is allocated This minimizes the amount of memory fragmentation in the program Should NETCELL be unable to allocate additional storage the simulation terminates with an out of memory error Under the above representation memory usage is bounded by the total number of packets and cohorts existing in the network at any given moment The number of packets should be a few times smaller than the product of the total number of destinations and the total number of cohorts existing in the network at any given moment because the typical cohort should only include packets for a fraction of the destinations The movement of traffic from cell to cell is governed by the flow density relationship for each cell This relationship is defined by a set of parameters associated with each arc By default the flow density relationship for an arc is assumed to be the triangular flow density relationship shown in Figure 1 This is the model used in reference 2 It is also possible to use general forms of the flow density graph as demonstrated theoretically in reference 4 The general flow density relationship 9 should then be describ
9. enlarging the view there may be no portion of the network visible In the window To see the network use the scroll bars to move around the window and locate the network THEME Windol Zoom In amp Zoom Out 3 36 Figure 18 The Options Menu 10 2 4 Windows menu The windows menu allows the user to manipulate the display of the various NETVIEW windows Selecting any of the show commands brings that window to the front making it visible if necessary The windows can be rearranged or closed using the title bar and close box or close menu in the top left corner of the title bar if desired They can be redisplayed and brought to the front using the windows menu Windows Show Network Show Curves Show Table Show Arc Selections Figure 19 The Windows Menu 37 References 1 DAGANZO C F 1994 The cell transmission model A dynamic representation of highway traffic consistent with the hydrodynamic theory Trans Res B 28 3 in press 2 DAGANZO C F 1994a The cell transmission model Network traffic Trans Res B accepted 3 LIN WEI HUA amp C F DAGANZO 1994 Technical Description of NETCELL General Framework and Data Structure 4 DAGANZO C F 1995 A Finite Difference Approximation of the Kinematic Model Trans Res B Vol 29B No 4 pp 261 276 38 Appendix 1 Flow Chart for NETCELL Start Read input Event type Open files Incident Setup arcs change cells ev
10. primarily by the amount of available memory Program limitations are discussed in the section on computational requirements below The simplicity of the theory has allowed us to develop an efficient code with easily verified building blocks Formulas are estimated for run time and memory usage Part 1 of this report describes the NETCELL program in detail including its input and output files The postprocessor program NETVIEW is described in part 2 the NETVIEW User s Guide As a prelude to our presentation the next section provides a glossary of terms Section 3 covers the internal structure of the program It describes the cell representation of a freeway network and the strategy for storing all the data in the memory Section 4 describes the simulation algorithm and its memory and computational time requirements Section 5 describes the file structure the input and output processes for the program and section 6 provides a simple example Sections 7 through 10 discuss the installation and running of the programs and the interface of the NETVIEW viewer program 2 Glossary of Terms The following is a set of terms frequently used in this report or in the program NETCELL and their definitions The symbol associated with the term is included in the parenthesis Arc k a homogenous roadway segment without entrances or exits characterized by its length miles free flow speed mph jam density vpm and maximum flow vph Cell i the s
11. roadway characteristics are attached to cells and not to links as would be conventional The cell characteristics are uniquely determined by the clock step 8 as is shown below For a clock step of 5 time units the length of each cell is defined to be v The number of cells used for arc k is the positive integer m that is closest to di 6 vgk This introduces a small change in the effective length of the arc which can be reduced by reducing Flow kok kjk Density Figure 1 Assumed flow density relationship The parameter qm becomes a cell specific constant that denotes the maximum number of vehicles that can either enter or exit the cell in one clock step For a cell i of arc k the constant is Q gm x This constant can be changed in the middle of a simulation run if we specify that an incident reduces the capacity at a point in the arc that corresponds to the cell s location The parameter k becomes a cell specific constant denoting the maximum number of vehicles that can be present in a cell at any given time For cell i of arc k the constant is N kj vgy The parameter w is represented in the cell transmission model by the dimensionless cell specific constant 0 tw VE ky where cell i is assumed to be in arc k The default a for an arc is calculated as 8qy kjk VeK 84m k 3 1 The network representation In this implementation of NETCELL the connectivity of the network is embedded in the r
12. such as the formation propagation and dissipation of queues In addition to the technical description of the internal simulation engine the report also details the installation of the package input requirements computational and memory requirements and limitations Thus it can serve as a user guide as well The NETCELL simulation package consists of two components NETCELL the simulation model itself and NETVIEW a graphical postprocessor for displaying output files from NETCELL NETCELL is a macroscopic simulation program in which vehicle quantities are treated as continuous variables Vehicles are advanced in a way consistent with the hydrodynamic theory of traffic flow Unlike most existing traffic models NETCELL preserves rigorously the first in first out FIFO discipline for multicommodity network traffic flows This unique feature is critical for studying freeway ramp metering and other control strategies and for evaluating the performance of these strategies The input of NETCELL consists of four parts describing the network geometry the routing information any incidents and the Origin Destination inputs In addition to the traditional input parameters NETCELL also allows a user specified piecewise linear flow density relationship This feature could enhance the realism in modeling wave propagation on freeways NETVIEW is a graphical windowing program and is available for two platforms the Apple Macintosh and Microsoft Wind
13. table is a ODTIME line specifying the starting time for the table followed by one ODROW line for each origin with each line containing demand values for each destination The format of the lines is ODTIME time This sets a start time for a new OD table The time does not have to be within the simulation start and end times but must be in the same units There is an implicit ODTIME 0 line at the beginning of the OD table specification section This allows the user to specify an OD table which will be used from the beginning of the simulation until such time as another OD table start time has been reached ODROW origin dest dest destp The ODROW line sets the traffic demands from the specified origin to each of the destinations in the network The time unit should be the same as that used to define capacity and time As for the route choice coefficients the order of demands is assumed to correspond to the order in which destination nodes occur in the geometry section of the input file Similarly if there are not as many parameters are there are destinations an error message is displayed There should be the same number of ODROW lines following an ODTIME line as there are origins in the network but this is not required The order of the lines within a table is not significant as the origin parameter is used to determine which line in the OD matrix is being defined Nor do the lines need to be directly sequential There may be on
14. table windows 32 Rrc Selection Selected Arcs Remove Enter Arc Remove Clear All Add All Cancel Figure 13 The Arc Selection Window To add an arc to the selection list the user enters the arc number in the arc number entry field in the top right of the arc selection window and clicks on the Add button If the arc number entered is valid the arc will be added to the selection list and the list on the left will update Arcs appear in the list in the same order as they appear in the input file To remove an arc the user enters its number in the entry field and clicks the Remove button If the arc was selected it is removed from the list The Add all button selects all arcs in the network while the Clear All button deselects all currently selected arcs The network curve and table windows are updated only when the user indicates they are done adding and removing arcs from the selection list This is done in one of two ways Clicking on the Done button or pressing the return key will update the arc selections and close the arc selection window The curve and table windows will update to reflect the new arc selections If the user would like to see the updated curves and table they can click on the Apply button This updates the arc selections and the other windows but keeps the arc selection window in front to allow the user to make further changes to the arc selection list The
15. x y coordinates of the nodes in the NETCELL input file as described in section 3 1 Since these coordinates are not necessary to run the NETCELL simulation the network representation may not correspond to the actual network 31 E Network View Se S C e Figure 12 The Network Window The network is initially scaled to fit in the default network window The window is resizable and scrollable and the network can be enlarged or reduced The Option menu contains two commands Zoom In and Zoom Out Zoom In scales the network up by a factor of two while Zoom Out reduces the network size by a factor of two If the network nodes do not have reasonable coordinates this window may appear blank The network window provides a view of the network only Itis provided as an aid to the user in selecting arcs of interest to examine When the arc selection list is updated the user picks Apply or Done in the selection window as described below the selected arcs will be displayed in red on color monitors 10 1 2 The arc selection window The arc selection window is where most of the user interaction in the NETCELL Viewer takes place On the left is a scrolling list of selected arcs Initially this list is empty Using the number entry field and the buttons on the right the user adds and removes arcs from the selection list The selection list in turn determines the contents of the curve and
16. 7 x It may run on system 6 machines but this has not been tested Both programs are fat binaries and run in native mode on both 680x0 and powerPC systems The default memory partition is set to 1 megabyte but this may not be suitable for the simulation runs any particular user may want to do The memory use of NETCELL is highly dependent on the number of cells and the number of cohorts in the simulation and is difficult to predict apriori If running NETCELL produces any out of memory error messages increase the memory partition and retry the simulation To increase the memory on either program select the program icon NETCELL or NETVIEW and select the item Get Info from the File menu of the finder Increase the amount of memory allocated to the program in the preferred size box to some larger number 82 Installation under windows Insert the NETCELL distribution disk into the floppy drive Either at the DOS prompt or using the Windows file manager create a subdirectory called NETCELL 28 or something similar This can be in a nested subdirectory if desired Copy the file NETZIP EXE from the floppy to the new NETCELL directory This file is a self extracting archive file At the DOS prompt type NETZIP This should expand the file and create all the files in the NETCELL simulation package Once the file has been expanded the NETZIP EXE file may be deleted The NETCELL simulation package has been tested under windo
17. An Example Network NETVIEW User s Guide 7 Introduction to NETVIEW 8 Installing the NETCELL simulation package Installation on the Macintosh Installation under windows Running NETCELL Running NETVIEW 10 1 NETVIEW display windows 10 2 NETVIEW menus 8 1 8 2 9 10 11 12 1 References Appendices Flowchart for NETCELL 13 15 15 23 25 28 28 28 28 29 30 31 35 38 39 39 Or WN Flowchart for Update Straight Flowchart for Update Diverge Flowchart for Update Merge Sample Input File for the Example List of Figures Assumed flow density relationship Generalized flow density relationship Cell Type specifications Merge and Diverge Cells in a Junction User specified flow density relationship Trapezoidal q k curves with wave speed greater than the default value The Computation of Cell Travel Time Ti t Locataions to Take Cumulative Counts Roadway Geometry for the Example O K Relationship for Every Arc in the Example Travel Time Ti t For Arc 1 At Timet The Network Window The Arc Selection Window The Curve Window The Table Window The File Menu The Edit Menu The Options Menu The Window Menu vi 1 Introduction This report describes the NETCELL simulation package a pair of computer programs that implement the cell transmission model Daganzo 1994 1994a programmed in C The cell transmission model describes the dynamic evolution of multicommodity traffic over a freeway netw
18. CALIFORNIA PATH PROGRAM INSTITUTE OF TRANSPORTATION STUDIES UNIVERSITY OF CALIFORNIA BERKELEY The NETCELL simulation package Technical description Randall Cayford Wei Hua Lin Carlos F Daganzo University of California Berkeley California PATH Research Report UCB ITS PRR 97 23 This work was performed as part of the California PATH program of the University of California in cooperation with the State of California Business Transportation and Housing Agency Department of Transportation and the United States Department of Transportation Federal Highway Administration The contents of this report reflect the views of the authors who are responsible for the facts and the accuracy of the data presented herein The contents do not necessarily reflect the official views or policies of the State of California This report does not constitute a standard specification or regulation May 1997 ISSN 1055 1425 The NETCELL Simulation Package Technical Description Randall Cayford Wei Hua Lin and Carlos F Daganzo Department of Civil Engineering and Institute of Transportation Studies University of California Berkeley CA 94720 1720 Abstract This report describes the NETCELL simulation package NETCELL is a freeway network simulation program based on the cell transmission model which captures the dynamic evolution of multicommodity traffic over a freeway network with three legged junctions in a way that is consistent wit
19. Si ft of vehicles in S heading for cell jd to cell j Rj max of vehicles that can be received by cell Riad max of vehicles that can be received by cell j j 1 Update inflow j Move buffer2 and outflow to new cohort Move cohort to cell j 4 41 Appendix 4 Start curve type calcluate Sending flows Sj and S j 4 calculate Receiving flow R j Tranfer cohorts If Packets in buffer Move buffer to new cohort Move cohort to cell j Update inflow j outflow j outflow End Flow Chart for Update Merge Procedure Use wave speed Use 2 regime copy Sj from cell i S j 4 from cell Update R S etd amp free cohort copy Sj from cell R S j44 from cell j Update R S etc opySj 1 fro cell j44 R Sj from cell Update R S etc Update R S etc Si max of vehicles that can be sent from cell S4 max of vehicles that can be sent from cell TE R max of vehicles that can be received by cell Cm merge coefficient forcell 42 Appendix 5 Sample Input File Used in Example KKK KK KKK ck ck ck ok KK KKK KKK kk kk kk ck kk kk kk kk Sk kk KKK KKK KKK KK KKK KKK KKK SIMULATIONCONTROLPARAMETERS KKK KKK KKK ck ck ck KK KKK KR KKK Sk kk kk kk kk Ck Ck kk kk ck ck KKK KKK KKK KKK KKK KK KK KKK simulation start and stop times seconds TIME 0 1250 clock tick length seconds CLOCK 5 cohort size threshold
20. Suggested data range 0 0 001 EPSILON 0 000001 ENDCONTROLS CkCckckckckckckckckckckokckckckckckckckckckckckckckckckckckckckckckc kk ROADWAYGEOMETRY KKK KKK KKK KKK KK KKK KKK KKK KKK KKK KKK KKK Node information 4 columns 1 0 Q9 4 node node x M number type coord coord NODE 0 1 10 40 NODE 1 0 20 40 NODE 2 0 30 20 NODE 3 0 30 60 NODE 4 2 50 20 NODE 5 2 50 60 Arc information 9 columns SUE 2 3 4 5 6 7 d down arc upstream stream length speed capacity jam number node node density B miles mi sec veh sec veh m ARC 001 2 5 01667 8 144 ARC 112 1 25 01667 8 144 ARC 213 1 25 01667 8 144 ARC 3 2 4 1 25 01667 8 144 ARC 4 3 5 1 25 01667 8 144 ENDGEOMETRY Custom curves for arcs if any Lines of form Arc number type type spefic parameters ENDCURVES Route choice coefficients for arcs emanating from a diverge junction of columns of destinations 1 dest i is the ith downstream node with 1 node type arc route choice coefficient 43 number dest 1 dest 2 dest 3 dest 4 DIVERGE 0 1 1 0 0 0 cock ck kk kk kk kk kk ke ke ke kk ko kokokokckckockckckckckck TRAFFICDEMAND KKK KKK KKK KKK KKK KKK KKK KKKKKKKKKK OD table for time 0 on Origin destination 1 destination 2 ODROW O 4 4 OD table for time 30 ODTIME 30 ODROW 0 4 4
21. TCELL vehicle quantities are real valued In certain cases the size of a cohort can be very tiny with a size of for instance 0 0001 At any clock tick if the size of a cohort residing in a cell falls below this user specified threshold the cohort will not be transferred alone at the current time Instead it will wait till some future time to be transferred together with other cohorts This is a real number and if the line is omitted the default value is 0001 OUPUTOCC b This line controls the production of the cell occupancy file The parameter b can be either 0 or 1 A value of 1 will cause NETCELL to write the cell occupancies for each clock tick to the TRC file See the section below for more information about this output file This line is optional and the default value is 0 ENDCONTROLS This ends the simulation control parameters section and must be the last line in the section 16 5 1 2 Road geometry The data in this section specifies the geometry of the network It ends with the ENDGEOMETRY Keyword Valid lines are NODE nodenum type x y Information about the nodes is used to set up the connections between arcs After the arc list is constructed the node information is discarded before the simulation begins The nodenum parameter must be 32 767 and must be an integer The type parameter can be 0 1 or 2 A type 0 indicates an ordinary node type 1 indicates an origin node and type 2 indicates a destination node The v
22. a is contained in a single text file whose name can be chosen by the user but whose extension must be INP The input file consists of five sections containing data defining the simulation parameters These sections are described in detail below and a sample input file is provided in appendix 5 There are six sections to the input file the control parameters the geometry information the curve specifications the routing information the OD table specifications and the incident information The sections must occur in that order and are separated by a keyword marking the end of the section Sections may be empty but the appropriate keywords must still appear Within each section input lines start with a type keyword followed by a variable number of parameters on a single line Parameters are separated with one or more spaces or tabs With the exception of the OD tables order of lines within a section is unimportant Keywords are always in all capital letters Any line not starting with a recognized keyword is treated as a comment line and ignored In general if the same parameter is multiply defined the last definition applies and earlier definitions are discarded After the six sections the input file may have a single line with the keyword ENDINPUT This tells NETCELL to stop processing the input file The ENDINPUT line may be followed by additional comment lines which are not read at all 5 11 Simulation controls This section defines the overa
23. alled is updateMerge we first calculate the overall flow s with the equations given in either section 2 2 or section 2 3 in Daganzo 1994a With these amounts as targets which should be met to within a tolerance of units and using a FIFO discipline the specific cohorts and packets to be moved are identified and transferred The tolerance level is needed to limit the fragmentation of cohorts that can arise in certain instances it should be a small number specified by the user The updateDiverge procedure uses as targets the maximum number of vehicles that can be received by cells j and j 1 Rj and Rj 1 and the maximum number of vehicles that can be sent by cell i S These quantities are calculated with the formulas in section 2 4 in Daganzo 1994a with the old n t nj t and nj 1 t as inputs The procedure then sends packets in FIFO order to the appropriate downstream cell split if necessary as per the route choice constants big The process is stopped when one of the three targets is met to within a tolerance of The flowchart includes more details Note that a packet will be divided in two parts or three parts for diverges if it cannot exit a cell in its entirety Conversely two or more packets with the same destination will become a single packet whenever they enter a cell in the same interval the buffers in the flowcharts achieve this Note as well that our strategy makes no attempt to preserve FIFO within pac
24. alues x y are 16 bit integers Ixl ly 32 767 denoting the location of the node in a rectangular coordinate system They are used by the NETVIEW program for drawing a representation of the network These values must be present but need not have any real meaning For instance all nodes may be located at point 0 0 without causing any problems for either NETCELL or NETVIEW ARC arcnum upNode downNode length speed capacity jamDensity The input of roadway geometry is arc oriented Here each line contains the information about a single arc There are a total of 7 columns in the data entry The first three columns of this data entry define the network connectivity with arcs and nodes in a conventional way The arc number must be an integer and is used in many of the input parameter lines described below The upstream and downstream node parameters are valid nodenumbers from a NODE parameter line The NODE line need not appear before an ARC line which references it references are resolved after all the lines have been read and processed The program generates an error if an ARC line contains a reference to a node which is never defined before the end of the geometry section The fourth and fifth columns specify the length and free flow speed of the arc The length and speed of the arc are given in the same units of measurement for time and distance used elsewhere in the input file Both are real numbers We recall that the actual arc length used by
25. called something like convert soft returns to hard returns will result in breaking long lines in the text file This will generate input errors when the file is read by NETCELL The input file is described in detail in section 5 1 above That section describes how the file must be laid out and what the available input parameters are A sample file is shown in appendix 5 as well Once an input file has been created the user is ready to run the NETCELL simulation program The input file should be copied to the NETCELL directory folder before running NETCELL As discussed in section 5 the name of the file must end in the extension INP To run NETCELL under windows or macintosh double click on the NETCELL program icon under DOS type NETCELL This will start the simulation program NETCELL will prompt the user for the name of the input file and wait for the data to be entered and the return key to be pressed The user should enter just the initial portion of the name without the INP extension which is assumed If the file cannot be found by NETCELL the program will terminate with an error message IF the file is found the program will read it and start the simulation Errors in the syntax of the lines of the input file will cause the NETCELL program to terminate with a message indicating what the nature of the problem is If an input file is failing to read properly the lines should be carefully checked to be sure that all lines have the
26. convert the input data into their corresponding cell representation For the purpose of illustration we will discuss further the result of the conversion The first two parameter lines in the simulation control parameters determine the total steps of the simulation run i e 1250 0 5 250 steps or clock ticks Under cell representation arcs 1 to 4 will be represented by 15 cells and arc 0 by 30 cells The length of each cell can be calculated as 1 12 miles as well as the maximum occupancy N 12 and maximum flow Q 4 The prespecified incident is identified to be inside cell 55 one of the cells representing arc 1 which lasts for 100 clock ticks When the incident occurs Q for cell 55 will be dropped to 1 The total number of cells used for this simple network is 93 For the traffic demands or the departure rates there are four vehicles leaving the origin zone at every clock step Of these two are heading for destination node 4 and the other two for destination node 5 We can now run the simulation and specify the input file TEST INP when asked The simulation run covers a period of 1250 seconds or 250 clock ticks The arc travel time over time is stored in file TEST OUT As an illustration the time it takes to traverse arc 1 for vehicles entering the arc at time t is displayed in Figure 11 26 Flow vps Arc travel time seconds d mi sec 48 144 Density vpm Figure 10 q k curve for every arc
27. correct number and type of parameters Also check that all input lines start with a keyword and that the keyword is in all capital letters Lower case keywords are treated as comments and ignored As the program runs it prints the current clock at each step of the simulation to the screen This is informational only to let the user know where the program is in its execution and allow an estimate as to the completion time A run completed message is printed on the screen when the program terminates At this point NETCELL has produced an output file with a FLW extension which can be viewed with the NETVIEW viewer application 10 Running NETVIEW To run the NETVIEW program double click on the NETVIEW program icon 30 NETVIEW is a viewer of NETCELL output files only it is not an input processor in any way NETCELL produces three output files the arc cumulative flow file the arc travel time file and the cell occupancy file The NETVIEW program uses the arc cumulative flow file which always has a file extension of FLW as its input See section 5 2 for detailed information about the format of the arc cumulative flow file and about the other NETCELL output files When NETVIEW starts it presents a standard file open dialog box The user must have already run NETCELL and have an arc flow file available When the open dialog appears select the FLW output file and click on the OK button NETVIEW will attempt to open and read the selected file
28. d by a single arc and each of the diverge branches by two arcs in series We assume that q k relation depicted in Figure 10 holds for all links and 50 of the traffic goes to each destination Initially the upstream link is assumed to be operating at capacity when a temporary incident Destination 1 8 Destination 2 Figure 9 Roadway geometry for the example 25 partially blocks one of the diverge branches The incident lasts for a certain period of time until it is cleared and traffic gradually returns to normal Node 0 represents the origin and nodes 4 and 5 represent the two destinations respectively The roadway geometry is governed by four parameters the capacity qx density kj free flow speed vgj and a wave coefficient which is universal for all arcs The capacity for each arc is 8 vps the jam density 144 vpm and the free flow travelling speed 01667 miles sec approximately 60 mph The wave coefficient is calculated to be 0 5 The length of a clock tick for this example is set to be 5 seconds It is also shown in the input data set there is an incident taking place inside arc 1 The location of the incident is 0 375 miles from the upstream end of arc 1 Traffic demands at origins route choice coefficients and some other simulation control parameters are also given in this input data file We also specify that the output file for occupancy data should be created The first step in the simulation run is to
29. e and no further lines will be processed 5 2 Outputs A set of two and optionally three output files will be generated by NETCELL when it runs All three output files are text files with a simple column based format which can be easily imported into software packages such as LOTUS QPRO or Splus to make various plots The three files are the arc cumulative count file the arc travel time file and the cell occupancy file The names of the output files are all based on the input file name with different extensions for each file type The acr count file has the extension FLW the cell occupancy file has the extension OCC and the arc travel time file has the extension OUT The arc cumulative count file is named similarly to the input file but with a FLW extension This file is used as the input file for NETCELL s companion postprocessor program NETVIEW As such its format is slightly more complicated than the other two output files The arc count file starts with a duplication of the input file All lines are echoed from the input file to the arc count file as these are processed including all comment lines This allows the program NETVIEW to read the arc count file alone and reconstruct the network and simulation parameters Including the comment lines allows the user to easily rerun the simulation using the arc count file as a new input file The input section of the arc count file ends with an ENDINPUT keyword line This line is added if ther
30. e or more comment lines between ODROW lines If no line for a particular origin appears in the table entry that row of the OD matrix is set to zero If multiple lines 2 1 for a single origin appear in the same table entry the demands in the last line are kept and previous values are discarded Warning messages will be printed for lines with fewer values than destinations lines left with default values and repeated lines for the same origin ENDODTABLES The end of this section is marked with the ENDODTABLES keyword line 5 1 6 Incident information This section allows the specification of changes to the capacity and jam density of particular locations along an arc at arbitrary times The section ends with the keyword ENDINCIDENTS There is only one possible kind of parameter line INCIDENT arcNum distance startTime endTime maxFlow The location of the incident is specified as lying a specific distance along the indicated arc For instance an incident might lie 5 miles from the start of arc 1 If the distance specified is longer than the length of the arc the incident is ignored The distance along the arc is used to determine which cell contains the incident The new capacity value will apply to that cell alone The start and end times set the period within which the changed capacity is valid They are in the same simulation time units as all other time parameters The maxFlow parameter is the new value for the cell capacity This val
31. e specified at particular points in an arc as if an incident had occurred Although we shall refer below to incident events it should be understood that the procedure can also be used for recurrent capacity reductions such as metering of certain locations Incident events are discussed in the section below on the simulation event system Cells are stored as a linked list attached to the arc They store traffic flow information and occupancy only Unless a cell has a cell specific set of flow parameters because of an incident there is no information about the physical facility stored in the cell itself Where two arcs merge into a third the simulation uses a merge priority coefficient to determine the percentage of traffic allowed to enter the merge cell This value is stored in the downstream arc The table of route choice coefficients used to determine the percentage of traffic bound to a destination which takes each leg of a diverge is stored in a similar way in the upstream arc These values are used only by the merge cell and the diverge cell if any of the arc A considerable memory savings results from storing them in the arc rather than in the cell itself however Arc 1 Arc 0 O O Arc 2 diverge cell merge cell Figure 4 Merge and diverge cells in a junction 3 2 The traffic flow representation In addition to the above input data the simulation needs to be able to store the state of the system at every clock tick The state
32. e was no such line in the original input file The remainder of the arc count file contains one line per clock tick of arc inflow and outflow counts The output line contains values for every arc in the network for the clock period There are four values per arc the inflow to the arc for the clock interval the outflow from the arc for the clock interval the cumulative inflow to the arc and the cumulative outflow from the arc The values show one decimal place and are separated by spaces 23 Cumulative counts gt t dj t Time Time vehicle h ES b Time vehicle h enters cell i exits cell i Figure 7 The computation of cell travel time T i t The arc count file is used at the end of the simulation run to produce the second output file the arc travel time file The arc travel time file stores the travel times for each arc T t where T t is the time it takes to traverse arc i when the arc was entered at time t Under FIFO the time at which a vehicle will exit an arc entered at time t is the time d t at which the cumulative number of departures from the arc equals the sum of the initial arc occupancy and the cumulative count of arrivals to the arc in 0 t Thus Ti t di t t See Figure 7 This calculation applies to travel times for individual cells as well The only difference in the treatment of cells and arcs is the placement of the counting locations see Figure 8 The T t s can be used off line to reconstruct rou
33. ed by two continuous piecewise differentiable functions as shown in Figure 2 one for sending flow and the other for receiving flow The actual flow entering a cell is determined by the minimum of the sending flow computed from the occupancy of its upstream cell and the receiving flow computed from the occupancy of the cell itself This generalization has been implemented in NETCELL in an approximation form To specify the sending and receiving flow curves for an arc the user would need to supply as input n points of data for flow and density as shown in Figure 5 where n 6 The flow density curve is then constructed by joining two neighboring points with line segments The resulting curve should be such that the absolute value of the slope of each line segment is less than the free flow speed An initial point with coordinates 0 0 and a final point with coordinates jam density 0 are assumed for the ends of the curve Flow kok kjk Density Figure 5 User specified flow density relationship As the simulation runs traffic enters the system at origin cells and exits the system at destination cells The generation and destruction of traffic flows is determined by a single global origin destination OD table of OD flows While at any given point of time there is only one OD table the table may change over the course of the simulation Time specific OD tables can be defined in the input file as explained below 3 3 Event representat
34. ell within reason for many desktop computers and low end workstations The computational time requirement for a simulation run in the worst case is on the order of SMADd where S is the simulation length in number of clock ticks and d is the average cell delay d 1 SMA Y Y d t in clock ticks There are non trivial disk storage requirements for output files as well The section below on output files has detailed descriptions of the various output options available The most detailed file the cell occupancy file has a storage requirement of 5MA per clock tick In the example above this is 75 000 bytes per clock tick If the simulation covered an 8 hour period with a 5 second clock the output file would occupy 432 megabytes A 2 hour simulation would require 108 megabytes 14 5 File Structure Input and Output Processes The input and output files are related via DOS derived file extensions The are potentially four files used by NETCELL characterized by four different extensions The INP file is the input file for the simulation run Two output files are always produced the arc flow file which has a FLW extension and the arc travel time file which has a OUT extension A file containing cell occupancies can be produced if that option is selected in the input file The cell occupancy file has a TRC extension All filenames are based on the input file name differing only in the extension 5 1 The input file INP All input dat
35. entation does not use this feature extensively it provides a easily expandable interface for future enhancements This might include such things as time dependent changes to the route choice coefficients or time dependent flow density relationships e g due to weather 4 The Simulation Algorithm Memory and Computational Time Requirements 4 1 The simulation algorithm Reference 2 proposed a cell transmission algorithm with two major steps 1 calculation of the inbound and outbound flows by destination for all cells and 2 revision of the cell occupancies by time of entry and destination as per the flows calculated in step 1 When a cell is considered its packet and cohort information along with the same information for its downstream neighbor s is updated based on the flow between the two or three cells The change in state due to the inflow is realized automatically when its upstream neighbor s are considered A flowchart of the whole program is shown in appendix 1 The program initializes various structures and then reads the input file After the input file has been read the program creates the arc and cell lists the event list and the OD table 11 Then the program enters the simulation loop where it remains until an end simulation event occurs The simulation loop advances the clock and then processes any events which have a time stamp less than or equal to the current clock tick After all events have been proce
36. ents Initialize OD table change Process events Cell type Ba Update origin Update first cell yum Update pdate Update merge Update cell occupancy Update Update middle straight cells Travel time calculation Collect Cell type cumulative counts Update destination Close files Update last cell Update straight End Update diverge 39 Appendix 2 Flow Chart for Update Straight Procedure Start curve type calcluate Use wave Sending speed flow Sj Use calculate 2 regime Receiving CUINE flow R Move cohort SetF to buffer min S j Rj Free Set S to size cohort of first cohort Set S to size of next cohort Move cohort Free to buffer cohort fract Transfer Move buffer Wero cohort to buffer to new cohort buffer Move cohort to cell j Update inflow j and outflow End 40 Appendix 3 Flow Chart for Update Diverge Procedure curve type calcluate Use wave Sending speed flow Sj Use calculate 2 regime Receiving curve lows Rj R j 4 copy Sj andS ig Update R S etd amp free cohort to buffer copy R jet S j of cohort Update R S etc decompose Sj into copy Sj and S i4 i lt S Rj Sj of cohort Update R S etc Update R S etc Transfer Move buffer1 buffers to new cohort Sj max of vehicles that can be sent from cell Sj ft of vehiclesinS heading for cell j Move cohort
37. epresentation of the arcs In the input file for a simulation run the network is described using nodes and connecting arcs The node information is used to set up the connections between arcs only and is discarded before the simulation begins The arcs themselves are not arcs in the traditional sense of links connecting nodes but are really just placeholders for information common to the cells which comprise the arc The network itself is represented by the list of cells belonging to each arc With three legged junctions and more than one cell per arc cells can only belong to one of three types ordinary type 0 diverge type 1 or merge type 2 see Figure 3 As shown in the figure ordinary cells are connected to one upstream and one downstream cell diverge cells are connected to two downstream cells and merge cells to two upstream cells Diverge cells are the last cells of arcs pointing to a diverge node and merge cells the first cells of arcs pointing away from a merge node This is illustrated in Figure 4 in which each arc is represented by four cells In addition the network must include special cells to represent origins and destinations These must be connected by a single link to an ordinary cell and have infinite N and Q The origin and destination cells are abstract cells and are not actually stored in the program but are synthesized by the simulation algorithm An arc in NETCELL must be connected to either zero one or two incoming
38. ese values based on either state information such as congestion levels or time The fromArcNumber is the number of the arc ending at the diverge it is used to identify the diverge junction to which this parameter line applies Only one set of coefficients is required for each diverge junction as by definition the coefficient for traffic flow taking the second arc is 1 the coefficient of the first arc For example for a diverge junction which is a part of a network with two destinations an input line would look like DIVERGE 0 1 0 8 0 3 This would specify that at the diverge cell at the end of arc 0 8076 of the traffic flows going to destination 1 and 3076 of the flows going to destination 2 would choose the diverge branch represented by arc 1 The parameter line is expected to have as many coefficient entries as there are destinations in the network The order of coefficients corresponds to the order in which destination nodes appear in the network geometry section of the input file If there are less coefficients in the line than destination nodes an error message will be printed and the program will terminate after all input lines have been read An error message is also produced if the set of coefficients is chosen erroneously such as to cause some flow to be routed to the wrong destination MERGE fromArcNumber toArcNumber coefficient The merge coefficient denotes the fraction of vehicles which come from each approach in a merge j
39. h the hydrodynamic theory of highway traffic NETVIEW is a graphical postprocessor for viewing NETCELL output files This document discusses implementation of the programs in detail including the cell representation for a freeway network with three legged junctions data and file structures inputs and outputs and some key algorithms developed to model traffic progression in junctions The memory and computational time requirements for the program are also estimated An example for a small network with a single origin two destinations and a single diverge junction is included This report also includes a user s guide to the NETVIEW program The NETCELL program is based on a prototype program written in 1994 This version incorporates some enhancements to the model and memory handling improvements to allow NETCELL to model very large networks This version of the NETCELL program should be useful for use as a research and engineering tool Keywords traffic simulation traffic flow model transportation network traffic congestion management dynamic traffic assignment Executive Summary This research report provides a technical description of a computer simulation package NETCELL which was designed as a research tool for studying traffic flow over a large scale network NETCELL was developed based on the cell transmission model a multicommodity traffic flow model especially powerful in capturing the transient behavior of freeway congestion
40. in the example 250 200 150 100 50 0 50 100 150 200 8 250 Time seconds Figure 11 Travel time T t for arc 1 27 7 Introduction to NETVIEW The NETCELL simulation package consist of two programs NETCELL the simulation model itself and NETVIEW a postprocessor for viewing an output file from NETCELL NETVIEW takes as input the FLW output file from NETCELL and allows the user to examine the cumulative flow time curves and the simulation occupancy counts for any selection of network arcs Curves and tables may be printed and the simulation data or a subset may be saved in a format compatible with spreadsheets or statistical analysis packages for further analysis NETVIEW is a graphical windowing program and is available for two platforms the Apple Macintosh and Microsoft Windows 8 Installing the NETCELL simulation package Installation of the NETCELL simulation package is very simple There are only two files one for each executable There is no installer program as installation is straightforward enough not to warrant one 8 1 Installation on the macintosh Insert the NETCELL distribution disk into the floppy drive On the hard disk create a folder called NETCELL or something similar This can be in a nested folder if desired Drag all the files from the floppy to the new NETCELL folder The NETCELL simulation package has been tested on both 68k macintosh systems and on powerMac systems running system
41. ion While NETCELL is primarily a clock based simulation model it does contain a mechanism to allow event driven changes to the state of the system The program maintains a list of events each of which has an associated trigger time Events are processed at the start of the clock interval greater than or equal to the event trigger time Events are processed before any cell flows are calculated for that clock tick While the event system is a general mechanism there are currently only four event types defined Two are trivial the start simulation event and the stop simulation event while the other two change the state of network These are the incident event and the OD table update event The incident event changes the capacity of one cell As the name implies it is intended to model accidents and similar occurrences though it can also be used to model other events which affect capacity on a time basis such as road work or even signals The OD table update event changes the global OD table used to generate traffic by all cells in the network The input file may contain multiple OD tables each with an associated start time When the system clock reaches the start time of an OD table that table replaces the previous OD table and the simulation continues An OD table remains in effect until the next OD table start time As mentioned above the event system is a general mechanism for allowing time dependent system changes While the current implem
42. kets and cohorts only across packets and cohorts This introduces an error but one which should be comparable with the small length of a clock step 4 2 Limitations memory and computational time requirements This section describes some of the size limitations on various parameters and attempts to give some indication of the likely hardware requirements This is difficult due to the dynamic nature of the use of system resources and only an approximate calculation can be done Many of the indices used in NETCELL are stored as integers On most small memory machines such as desktop computers this is a signed 16 bit quantity so the maximum allowable value of these indices is 32 767 In particular we note that a 13 network can have only 32 767 arcs Each arc can have no more than 32 767 cells There can be 32 767 origins and 32 767 destinations in the network as a whole These are however limits which are unlikely to be a problem Likewise the number of cohorts and packets is limited by the 32 bit pointers that form the linked list This number is so large that the maximum number of cells and cohorts is effectively limited by the amount of available system memory The memory requirements are dominated by two major components the size of the network and the maximum number of cohorts in the system at any given time The former is determined at the initialization stage of a simulation run The latter is dynamic varying with system evolu
43. ll simulation parameters It must be the first section in the input file and it ends with the keyword ENDCONTROLS The order of lines is not significant Possible parameter lines are TIME be This line specifies the beginning and end times b and e of the simulation run It is 15 anticipated that these times specify seconds or hours based on a 24 hour clock with 0 as midnight but this need not be the case It is important to note that all times throughout the input file are in the same units Thus if the beginning and end times are in seconds the clock tick arc speeds arc capacities and origin destination flows must all also use seconds The memory usage of NETCELL is independent of simulation length A long simulation period specified here will not result in running out of run time memory but it will generate big output files UNITS Seconds This line is optional and specifies the unit of time that the user has chosen The information is used for labelling the time axis when displaying flow curves in NETVIEW It is not used in any way in the simulation itself CLOCK d This line specifies the discrete time interval between clock ticks in the time units chosen by the user It may be whole or real valued This value in conjunction with the arc lengths and speeds determines the number of cells in an arc The size of this value has a major impact on the memory usage of the simulation There is no default value EPSILON e In NE
44. m icon for the NETVIEW program in the same way The path and program name should be C NETCELL NETVIEW EXE The working directory for NETVIEW may be set to the NETCELL directory as well though this is not required At this point the NETCELL simulation package is installed and ready to run The memory use of NETCELL is highly dependent on the number of cells and the number of cohorts in the simulation and is difficult to predict apriori If running NETCELL produces any out of memory error messages the user may have to decrease the system resources used by other things This may involve quiting any background programs or in more extreme cases removing device drivers or other memory resident programs and rebooting the machine 9 Running NETCELL Once the programs have been installed the NETCELL simulation program is ready to be run Before running NETCELL the user must create an input file The 29 input file can be created in any word processor or text editor If using a word processor the file must be saved as a text file which usually requires using a special technique when saving the file Consult the user s manual for the specific word processor for information on how to do this Most word processor wrap long lines to within the document margins Some of the lines in a NETCELL input file may be very long Any long lines must be on a single line and not wrapped Saving as text will usually not wrap lines although an option usually
45. mallest component of the network in the cell transmission model representing section of an arc that is covered in the time between clock ticks 8 time units at the arc s free flow speed Although longer cells could be used this degrades the accuracy of the simulation and is not allowed in this version of NETCELL Cell length the same for all the cells in one arc this distance meters should be covered in one clock step at the arc s free flow speed Cell occupancy n t a non negative real valued state variable indicating the number of vehicles in cell i at time t Clock tick time instant f at which the inbound flow outbound flow and occupancy of every cell are updated and recorded The time between clock ticks is d The aforementioned flows correspond to the interval t t d Cohort Snjq t the number of vehicles residing inside cell i at time t that entered the cell in the time interval t t 5 for t lt t Companion cells cells that share the same upstream cell or the same downstream cell Companion links arcs k and ck links arcs that share a common cell node Each link arc can have at most one companion link arc Current time t same as current clock tick Destination zone d a single node or a single cell in cell representation from which vehicles leave the network Free flow speed v a cell specific value equal to the free flow speed of the corresponding arc oy Inbound flow y t
46. n it The cohort stores the information about the vehicles in the network at the cohort level A cohort is an aggregate of the vehicles in its packets and stores only the total number of all vehicles in the cohort the cohort size and the cell entry time or more specifically the lower end of the cell entry interval and a pointer to its packets The packet list describes each cohort s component packets It divides vehicles inside a cohort into smaller units according to their destinations A packet stores only three items its size its destination and a pointer to the next packet in the cohort The order in which packets occur in the packet list for a cohort is not significant NETCELL maintains the FIFO order at the cohort level only Traffic bound for alternate destinations within a cohort are treated as uniformly distributed within the cohort The linked list scheme allows vehicular progression to be traced easily and efficiently When a cohort moves from one cell to another without merging with other cohorts we only need to update the pointers in the cell cohort lists and the cell entry time for that cohort The cohort packet list can be left untouched When several cohorts enter a cell at the same time they will merge into a single cohort As a result a new cohort will be formed and the old cohorts and their packets will be deleted To increase the speed of the simulation and to avoid excessive memory fragmentation the program maintains
47. ne The number of pairs must match the points parameter All the values should be non negative real numbers and the x values should be smaller than the jam density for the arc The x sequence must be strictly increasing while the y sequnce should be unimodal no multiple maxima An initial point at 0 0 and a final point at jam density 0 are added by the program The pairs produce a line segment graph of the type shown in Figure 2 starting at the origin and ending at the jam density The information provided with this option overides the capacity provided for the arc in the geometry section The maximum slope of the linear segments in absolute value cannot exceed the free flow speed specified for the arc in the geometry section ENDCURVE This section ends with the ENDCURVE keyword 5 1 4 Routing information This section specifies the behavior of the traffic flow at merge and diverge junctions It ends with the ENDROUTING keyword There are two possible parameter lines as follows DIVERGE fromArcNumber toArcNumber c1 c3 CD The route choice coefficients ca denote the proportion of traffic flow heading for destination d that chooses one particular downstream leg of the diverge junction The arc number of this leg is identified as the toArcNumber parameter In the current implementation the coefficients are time and situation invariant A later version may enhance the modelling of diverge junctions to dynamically adapt th
48. oefficient ba t and baa t destination and time specific parameters for the companion arcs of a diverge junction k and ck specifying the proportions in which n t is split at the diverge in time interval t t 8 ba t bgg t 1 Transfer size threshold e an input parameter specifying the smallest cohorts transferable from cell to cell in the program Amounts under this threshold are not transferred This parameter is used by the program to prevent the proliferation of cohorts without introducing appreciable error Travel time T t or T t the time it takes to traverse cell i or arc k for vehicles entering the cell the arc at time t Wave coefficient a a dimensionless constant representing the ratio of the backward moving wave speed to the free flow speed 3 Cell Representation and Data Structures Consider a freeway network represented in the conventional way as a graph with a set of directed arcs k a set of nodes and some connectivity information Each arc of this graph is associated with some key physical descriptors of the road segment it represents These include the segment s length d and four parameters free flow speed vf maximum flow or capacity qm jam density k y and a calculated backward wave speed w that jointly describe a triangular flow vs density diagram as that of Figure 1 These descriptors are assumed to be given in some consistent system of units e g meters and seconds miles and hours
49. of the system consists of the number of vehicles by destination and time of entry in every cell njq t the sum of these over d n t and the cell occupancy n t The njq t represent the number of vehicles with destination d to have entered cell i in time interval t t 5 that remain there at time t each such group of vehicles is called a packet The n t represent the number of remaining vehicles to have entered cell i in interval t 1 0 irrespective of destination each such group will be called a cohort Packet and cohort size information is necessary to maintain the first in first out FIFO discipline and to preserve the multicommodity nature of flow A section below will describe the algorithm for updating the state of the system Here we describe how the data are stored Instead of creating cell specific tables with packet and cohort size information the program dynamically allocates storage for cohorts and packets to trace the movement of traffic in the network The dynamic allocation of these structures eliminates the need for saving in each cell enough memory to store information pertaining to the maximum number of packets that could possibly be present in it this is important because the maximum number of packets in a single cell could be very high but at any given time t most cells in a network even a congested one will be underutilized Each cell maintains a list of the cohorts which are currently i
50. ork with three legged junctions in a way that is consistent with the hydrodynamic theory of highway traffic As such NETCELL is a purely macroscopic model in which vehicle quantities are treated as continuous variables Thus in this report the words number of vehicles should always be interpreted as designating a real number The NETCELL program is based on a prototype program written in 1994 Lin Daganzo 1994 This version incorporates some enhancements to the model and memory handling improvements to allow NETCELL to model very large networks A graphical postprocessor has been written as a companion to the simulation model Further testing and verification of the basic model has been done since the prototype was developed Additional error checking has been also added to NETCELL While the prototype was sufficient to show proof of concept and to confirm the validity of the theoretical model this version of the NETCELL program should be useful as a research and engineering tool The NETCELL simulation program can handle networks with three legged junctions as described in the theory and includes a graphical postprocessor NETVIEW for viewing the simulation results Some error checking for syntax errors in input data entry is done but checking on logic errors has been purposely left out of the program leaving room for the user to explore applications beyond the limitations of the program The size of network which can be modeled is limited
51. ow and the outflow of the merge cell Function updateStraight calculates the flow of an ordinary link not part of a merge or diverge i e the outflow of a cell and the inflow of its downstream cell Function updateDiverge calculates the outflow of the diverge cell and the inflows of the two downstream cells The sequence in which the cells and their update procedures are evaluated is as follows For each arc in turn Step 1 if the arc has no incoming arcs call updateOrigin for the first cell and move to the next cell If the arc has two incoming arcs call updateMerge for the first cell and move to the next cell Step 2 if the current cell is not the last cell in the arc call updateStraight and move to the next cell Step 3 if the arc has no outgoing arcs call updateDestination and move to the next arc If the arc has two outgoing arcs call updateDiverge and move to the next arc If the downstream cell is a merge cell move to the next arc without processing this cell otherwise call updateStraight 12 When the last cell has been processed the overall occupancies n t are updated The logic of the five update procedures called above are briefly described below The updateStraight and updateMerge procedures are very similar and can be described jointly Let cell j be the downstream cell of the current cell i with cell i 1 as the companion cell for merges Using n t and nj t and nij 1 t if the function c
52. ows The output of NETCELL can be manipulated in NETVIEW with four display windows and four menus The windows are the network window which displays a graphical representation of the network the arc selection window which allows the user to select and deselect the arcs which are used to calculate results the curve window which displays flow time curves for the selected arcs and the table window which displays the cumulative counts and other information for the selected arcs The cumulative counts at user specified locations are not available as outputs in other existing traffic simulation programs The NETCELL simulation package provides a platform for evaluation of ITS improvements environmental impacts and dynamic control strategies It holds promise for the study of dynamic traffic assignment real time travel information and other areas in traffic flow modeling where a proper representation of physical queues is of paramount importance Part 1 1 2 3 3 1 3 2 3 3 Table of Contents NETCELL technical description Introduction Glossary of Terms Cell Representation and Data Structures The Network Representation The Traffic Flow Representation Event Representation 4 The Simulation Algorithm 4 1 4 2 5 5 1 5 2 6 Part 2 Memory and Computations Time Requirements The Simulation Algorithm Limitations Memory and Computational requirements File Structure Input and Output Processes The Input File Ouputs
53. slope of the trapezoid s right side remains to be determined This is done by means of the waveCoefficient parameter a which gives the absolute value of said slope in units of v This absolute value cannot be less than the default see Figure 6 In addition since the backward moving wave speed cannot exceed the free flow speed the upper bound of the wave coefficient must be 1 It is in reality serveral times smaller than 1 The lower bound of this parameter is determined by the values of the free flow speed jam density and maximum flow as specified in the ARC definition line For example suppose an arc has a jam density 180 vpm free flow speed 60 mph and maximum flow 1800 vph The lower bound of the wave coefficient is 0 2 The choice of the wave coefficient ranging from 0 2 to 1 in this case gives different shapes of the flow density relationship as shown in Figure 6 18 kok kjk Density Figure 6 Trapezoidal q k curves with wave speed greater than the default value OKCURVE arcNumber 2 points x1 y1 x2 y2 x3 y3 This form of the OKCURVE parameter line forces NETCELL to use a piece wise linear flow density curve specified by the user for the given arc The curve is specified by the provision of an arbitrary number of xy coordinate pairs denoting points along the desired curve in the same units used for the arc definition The points parameter in the above line indicates how many coordinate pairs follow on the input li
54. ssed the program updates the flows of each cell The algorithm traces each arc in turn in the order in which they occurred in the input file Cells are considered in spatial order along an arc The sequence in which arcs and cells are considered is unimportant for the algorithm except in the case of a cell directly upstream of a merge cell In that particular case the outflow of the cell is calculated as part of the inflow calculation for the merge cell In all other cases the outflow of the cell and the inflow of the downstream cell or cells are calculated together There are five different procedures called to update cells depending on the connectivity of the parent arc and the cell s position in the arc They are updateOrigin updateDestination updateMerge updateStraight and updateDiverge The flowchart in Appendix 1 shows the procedure which should be invoked depending on the arc and the cell position in the cell list Function updateOrigin calculates the inflow for a cell directly downstream from an origin cell Since an origin cell is an abstract entity and does not actually have a representation in the arc and cell lists this is a special case Similarly updateDestination is called for the last cell in arc which is directly upstream from a destination cell The three main procedures are illustrated by flowcharts in appendices 2 3 and 4 Function updateMerge calculates the outflows of the two upstream cells and the infl
55. te travel times and calculate time dependent shortest paths The travel times are stored as one line per clock tick with the start of the time interval at the beginning of the line followed by one value per arc Values are separated by spaces Production of the third output file the cell occupancy file is determined by the flag on the OUTPUTOCC line of the input file This line is optional and by default it is not produced as it can grow to very large size on even moderate sized problems The cell occupancy is output as a single line per clock tick with all the 24 current occupancy values for all the cells in the network on that arc cell cell arc arrival arrival departure departure counts counts counts counts Figure 8 Locations to take cumulative counts one line Cells appear in the output line in the order in which the arc to which they belong appeared in the input file Within the arc cells are in sequential spatial order upstream first There is no indication of the breaks between cells belonging to one arc and the cells belonging to the next Thus to use this file to do further analysis while possible may require some effort to reconstruct the arc to cell translations 6 An Example Network The example given in this section is a simple network with a single origin two destinations and a single diverge junction It is based on the sample INP file given in appendix 5 As shown in Figure 9 the upstream section is represente
56. the computer algorithm is the integer multiple of cell length clock tick interval times the flow speed that is closest to the specified value Thus short clock intervals result in a more accurate representation of the network NETCELL requires each arc to be represented by at least two cells This requirement can be met by shortening the clock interval The last two columns are used to specify capacity and jamDensity The arc input is sufficient to define a triangular flow density relationship of the form discussed in 17 section 3 2 The capacity is assumed to be in vehicles unit distance The jamDensity is in vehicles unit distance ENDGEOMETRY This section must end with a line starting with ENDGEOMETRY 5 1 3 Curve specifications This section defines any custom flow density relationships for the arcs in the network It ends with the ENDCURVE keyword There is only one possible input line the ODCURVE parameter line which can have two variants This section may be empty except for the ENDCURVE line OKCURVE arcNumber 1 waveCoefficient The second parameter in a OKCURVE input line is a type code specifying what kind of a curve is being defined for the arc specified by the arc number A type code of 1 indicates that the curve for this arc is a trapezoid such as those of Figure 6 When this option is used the information in the ARC definition line is still used to specify the parameters qq Vk and kj of Figure 6 so that only the
57. tion as the simulation progresses Almost all of the memory to store the network geometry is used to store the arcs and the cells Let A be the total number of arcs in the network The memory to store the arcs is 166 8D where D is the number of destinations in the network If M is the average number of cells per arc there are MA cells in the system which require 40MA bytes of storage So the total memory required to store the network geometry is on the order of 40M 166 8D A The total number of cohorts existing in the network can be estimated from the average cell delay in the system Let average cell delay at clock tick t be d t d t 1 MA X d t Let T max d t The total number of cohorts required is on the order of MAT The size of the packet list for a cohort is destination dependent In the worst case where we assume that each cohort is mixed with components heading for every destination the size of each cohort is 24 10D bytes The total memory required for cohorts are MAT 24 10D bytes The total memory requirement for the simulation is on the order of 10M ATD bytes As an example if we have a network with 1 000 arcs and an average of 15 cells per arc a maximum average delay of 2 clock ticks and a network with 50 destinations the memory to store the network geometry is approximately 1 2 megabytes and the memory to store the cohorts and packets is approximately 16 megabytes A total memory requirement of 18 megabytes is w
58. ue replaces the default value the cell normally uses When the incident ends the cell s capacity returns to the default value for the arc Incidents cannot be nested or overlapped within a single cell If an incident affects a particular cell from time 10 to time 40 and a nearby second incident affects the same cell from time 20 to time 30 the cell will use the value from incident 1 from time 10 to 20 the value from incident 2 from time 20 to 30 and the default arc value from time 30 on The remaining period for incident 1 is lost Because this may or may not be done intentionally the program issues a warning every time it happens The problem should not arise if the cell length is smaller than the physical separation between the incidents thus it may be removed by decreasing the clock step To model multiple incidents at the same location e g variable metering rates the input file should specify a sequence of non overlapping incidents at the same location An incident does not affect the free flow speed for the affected cell This should not be a cause for errors if the cells are small as required by the theory underlying NETCELL The geometric form of the flow density relationship for the cell with the incident will be identical in shape to the one chosen for the original except that it is truncated at the top 22 ENDINCIDENTS This section ends with the ENDINCIDENTS keyword The incident section is the last section of the input fil
59. unction when the supply of vehicles from each approach is not exhausted This entry is similar to that of route choice coefficients and is time invariant as well The user only needs to specify the coefficient with respect to one of the arcs upstream of the merge node The program will then identify its companion arc and assign the remaining fraction to that arc The default for all coefficients is 0 5 A warning message will be printed for merge junctions left with the default but the simulation will continue to run 20 ENDROUTING The ENDROUTING keyword ends the specification of route choice and merge coefficients 5 1 5 OD table specification This section specifies the generation of traffic flows in the simulation Traffic demand is represented as a two dimensional matrix with origin as the first dimension and destination as the second This is the only section in which the order of lines within a section is significant The two dimensional tables are input as a series of lines one for each origin with each line specifying the traffic from that origin to all destinations in the system Multiple OD tables may be specified for a single simulation run Each table has a starting time which NETCELL uses to update the OD table in use at any particular time step within the simulation The starting time for a table is specified in a parameter line which must appear before the origin destination lines to which it applies Thus the format for an OD
60. ws for workgroups 3 11 It may run under windows95 or under windows NT but no testing has been done It should work under windows 3 1 as well The NETCELL program comes in two versions one for DOS and one for windows The DOS program NETCELLD EXE uses normal DOS memory only and so is limited to problems which can run under the 640k memory limitation of DOS The windows version NETCELL EXE uses whatever windows resources are available to it so it can potentially simulate much larger networks Once the programs have been copied to the hard disk the user should create windows program icons for them Under the windows program manager create a new program group called NETCELL simulation package It can be saved in the NETCELL directory or in the windows directory In the new program group create a program icon for the NETCELL simulation executable To do this select New under the file menu of the program manager and click on program icon in the resultant dialog box Name the new program icon NETCELL Enter the full path and program name for the NETCELL program and click the OK button The path to enter should be C NETCELL V NETCELL EXE Select the program icon and select Properties under the file menu Under working directory enter the path for the NETCELL directory This will typically be C NETCELL This will set NETCELL to store its output and working files in the NETCELL directory when run Next create a progra

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