Overview of Simulation Logic

Overview of Simulation Logic This section provides a general overview of the logic/models employed to control the movement of vehicles within SwashSim.

Simulation Time Step SwashSim employs a 0.1-s simulation time step. That is, it updates the properties of all simulation objects (vehicles, signal control, etc.) 10 times per second.

Vehicle Entry Two types of vehicle entry distributions can be specified: Uniform or Random. For a uniform distribution, each vehicle will enter at the same headway, i.e., 3600 s/h divided by the entry volume (in veh/h). For example, an entry volume will result in a vehicle entering the network every 2 seconds. If the ‘Random’ distribution type is selected, two different distributions will be used, as follows:
 * Entry flow rate < 900 veh/h &#8594; Negative Exponential distribution
 * Entry flow rate ≥ 900 veh/h &#8594; Normal distribution
 * The standard deviation for the Normal distribution is calculated as: (Average Headway × Number of Lanes - MinArrivalHeadway) / 2.5
 * The minimum arrival headway is calculated as: DesiredHeadwayMultiplier × 1.1 + (StopGap + MinVehLength) / DesiredSpeed

Vehicles Any number of different vehicles can be included in the simulation. By default, the following vehicles are included: Because of the detailed vehicle dynamics and fuel consumption/emissions modeling employed in SwashSim (discussed later in this section), numerous vehicle characteristics must be specified for each vehicle included in the simulation. An overview of these inputs follows.
 * 2006 Honda Civic Si
 * 2005 Mazda 6
 * 2008 Chevrolet Impala LS
 * 2004 Pontiac Grand Am GT
 * 2001 Volvo S40
 * 2009 Honda Civic
 * 1998 Buick Century
 * 2002 Chevrolet Silverado
 * 2011 Ford F150
 * 1998 Chevrolet S10
 * 2004 Chevrolet Tahoe
 * Single-unit truck
 * Intermediate tractor + semi-trailer
 * Interstate tractor + semi-trailer
 * Tractor + double trailer

Base Info The base vehicle information includes: The default values can be changed under ‘Settings &#8594; Vehicles &#8594; Base Info’, as shown below. You can add or remove a vehicle by clicking on the green ‘+’ or delete a vehicle by clicking on the red ‘X’. Clicking ‘Save Changes to Memory’ will save any changes to the settings for your current simulation session, but they will be lost after closing the program. To save changes permanently to the vehicle database, click ‘Save Values to File’.
 * Vehicle name
 * Vehicle ID
 * Fleet type (automobile, truck)
 * FHWA classification
 * Maximum deceleration
 * Desired speed proportion. This value is an additional multiplier for a vehicle’s desired speed.
 * Length, width, and height
 * Weight
 * Wheel radius
 * Drag coefficient
 * Engine ID
 * Transmission ID
 * Minimum driver type
 * Maximum driver type

Engines Each vehicle needs to have engine characteristics specified for it. The engine information includes: The engine values can be specified under ‘Settings &#8594; Vehicles &#8594; Engines’, as shown below.
 * Engine label
 * Engine ID
 * Displacement
 * Strokes/cycle
 * Volumetric efficiency
 * Engine idle speed
 * Maximum engine speed

For each engine, a torque-engine speed relationship must also be specified. This input screen is accessed by pressing the ‘Torque Data’ button in the engine settings window, which will open a window as shown below.

Transmissions Each vehicle also needs to have transmission characteristics specified for it. The transmission information includes: The transmission values can be specified under ‘Settings &#8594; Vehicles &#8594; Transmissions’, as shown below.
 * Transmission label
 * Transmission ID
 * Drive axle slippage
 * Drivetrain efficiency
 * Differential gear ratio
 * Transmission gearing information, which includes:
 * Gear number
 * Gear ratio
 * Downshift speed
 * Upshift speed

Drivers For each vehicle generated into the network, a driver type is randomly assigned. By default, 10 driver types are included. Driver type ‘1’ corresponds to the most conservative type of driver and driver type ‘10’ corresponds to the most aggressive type of driver. The default settings are stored in the ‘DriverParms.xml’ file, which is in the ‘DataFiles_ModelParms’ folder under the program installation folder. Values specified for each driver type include: The default values can be changed under ‘Settings &#8594; Drivers’, as shown in below.
 * Driver ID
 * Desired headway multiplier
 * Desired speed multiplier
 * Desired acceleration multiplier
 * Desired deceleration multiplier
 * Desired maximum speed
 * Desired speed deviation. This is the ± range that the vehicle’s speed must be within of the desired speed when the vehicle is able to travel at its desired speed.
 * The percentage of each driver type in the traffic stream, which must total to 100%.

Desired Speed Tangents The desired speed for a vehicle is equal to the desired speed multiplier for the given driver type and the link free-flow speed. The desired speed multiplier values can be changed in the settings. The default values are given in the following table. The link free-flow speed is coded by the user in the network setup.

Horizontal Curves Forthcoming.

Car Following The car-following model is used to calculate an acceleration value for a vehicle in a follower mode. See Appendix A for more information.

Maximum Acceleration The approach used to model vehicle maximum acceleration in SwashSim is based on the vehicle performance theory and equations given in Principles of Highway Engineering and Traffic Analysis (Mannering and Washburn, 2012). See Appendix B for more information.

Lane Changing The lane-changing logic is described in Appendix C.

Two-Lane Passing Detail about the logic used for two-lane highway passing maneuvers is given in Appendix A. Below are screenshots of the associated parameter value input screens.

Desire to Pass (Oncoming Lane)

Acceleration Values (Oncoming Lane)

Acceleration Values (Passing Lane)

Emissions