13.4.4 Basic Geometry – Freeway Coding
13.4.4.1 Merge
Freeway entrance ramp merge areas are coded using connector lane change distance (LCD) in combination with a vehicle route. The effective merging area typically includes the entire auxiliary lane (or lane drop) to the farthest extent of the auxiliary lane taper and capture the full effective length used by vehicles. Vehicles in Vissim use the extra link length, when necessary, which more accurately models the use of the taper area. The merging or weaving section is typically one link with the number of lanes equal to the number of lanes on the main freeway plus the number of lanes merging onto the freeway. There can only be one connector downstream of the merge link or at the end of a lane drop section. There can be two connectors upstream of the merge link, one for the ramp link and one for the main freeway link. below shows an illustration of a merge segment.
Figure 13-3: Freeway Merge Coding
The modeler typically uses the LCD of the connectors to accurately code the merge/weave area. One of the following two options can be implemented to avoid unrealistic lane changes on the freeway mainline into the acceleration lane:
- In the Connector dialog box, confirm the “Lane Change” distance for the connector downstream of the merge link is longer than the length of the merge area.
OR
- Indicate “No Lane Change” for the appropriate lane, using the Link dialog box.
13.4.4.2 Diverge
Freeway exit ramp diverge areas (parallel) are coded similar to the entrance ramp merge areas. The effective diverging area includes the entire auxiliary lane (or drop lane) starting at the taper and continuing to the painted gore point. The diverge section is typically one link with the number of lanes equal to the number of lanes on the freeway plus the number of lanes diverging from the freeway. There can only be one connector upstream of the diverge link. There can be two connectors downstream of the merge link, one for the ramp link and one for the main freeway link. The LCD can be coded similar to that of the merge/weave area of entrance ramps such that the distance exceeds that of the diverge area.
Freeway exit ramp diverge areas (taper) are coded using one connector placed at the painted gore point connecting the main freeway link to the ramp link. The main freeway link may not be broken with a connector. An appropriate LCD is recommended to be assigned to the connector to give vehicles enough time to merge to the correct lane to reach the exit ramp. A LCd is assigned such that it reflects the driver expectation and typically matches the location of the first guide or overhead sign indicating the approaching off-ramp. below illustrates a diverge segment.
Figure 13-4: Freeway Diverge Coding
13.4.4.3 Weaving
Weave segments follow the same rules as merge and diverge areas. Appropriate LCD is assigned to connectors, so vehicles follow realistic weaving behavior. This can be based on guide sign placements or other driver behavior characteristics of the local area.
13.4.4.4 Managed Lanes
HOV managed lanes are coded by restricting certain vehicle classes from a lane in the link. It is best practice to code a separate HOV managed link display type to show which lane is coded as an HOV managed lane. These lanes can also be coded as an independent link if physically separated from the general-purpose lanes.