Transportation Systems Simulation - A Tutorial for Multi-Modal Simulation Using VISSIM/Freeway: Improving Results
After the first simulation, you may have encountered unusual behavior. VISSIM v7 has tools to adjust a few driving parameters.
Adjusting Driving Behavior[edit | edit source]
Every link and connector can have a unique Behavior Type. Under “Network Objects” Right Click “Links” and Select “Show List” (see Figure below). A new window will appear below the network editor. All Links and Connectors by default should be “Urban (motorized)” as the “LinkBehavType”.
TIP: VISSIM uses two types of models: Wiedemann 74 and Wiedemann 99.
- Wiedemann 74 is suitable for urban traffic and merging
- Wiedemann 99 is suitable freeway traffic with no merging
- You may need to change the “LinkBehavType” for some or all links/connectors
WARNING: Run the simulation first with default values. Only make the following adjustments if the first simulation does not accurately represent highway traffic behavior.
Wiedemann Model (Following)[edit | edit source]
- To adjust specific parameters for each Wiedemann model, Select Base Data > Driving Behaviors from the top toolbar (see Figure below)
- A new “Driving Behavior” window will appear below the Network Editor
- Right Click your desired CarFollowModType and select “Edit…”
- A new window will appear and only adjust the following parameters
“Following” tab and Car following model: Wiedemann 74
- Look ahead distance
- Look back distance
- Average standstill distance
“Following” tab and Car following model: Wiedemann 99 (Figure 44)
- Look ahead distance
- Look back distance
- CC0 (Standstill Distance)
- CC1 (Headway Time)
- CC2 (‘Following’ Variation)
TIP: Only change parameter values in small increments if necessary. The table below shows the default value and the Suggested Range for each parameter.
CC0 (Standstill Distance): the average desired standstill distance between two vehicles or the desired rear-bumper to front-bumper distance (feet) between stopped vehicles, as shown in the Figure below.
CC1 (Headway Time): is the gap (seconds) which a driver wants to maintain at a certain speed. The higher the value, the more cautious the driver is as shown in Figure 46. Desired Safety Distance = CC1 +CCO.
CC2 (Following Variation): is the longitudinal oscillation during following conditions. It defines the additional distance (feet) from the desired safety distance a driver allows before he intentionally moves closer to the car in front. (see Figure below)
Wiedemann Model (Lane Change)[edit | edit source]
- “Lane Change” tab and General behavior: “Free lane selection” or “Slow lane rule”
- Check the “Cooperative lane change” box (see Figure below)
WARNING: It is not recommended to adjust Lane Change parameters until all previous steps have been attempted.
After adjusting a few Driving Behavior Parameters, run the simulation again and calculate the GEH with your new Simulation volumes. Only adjust Driving Behavior Parameters one at a time and by 10% to 25%.