An option lane is a common geometric feature present in freeway exits, providing operational flexibility. As defined by the MUTCD:
“Some freeway and expressway splits or multi-lane exit interchanges contain an interior option lane serving both movements in which traffic can either leave the route or remain on the route, or choose either destination at a split, from the same lane.”
Source: Manual on Uniform Traffic Control Devices (MUTCD-2009)
Modeling freeway weaving – required inputs
The HCM freeway weaving methodology requires inputs related to lane-changing configuration, as follows:
- LCRF = minimum number of lane changes that a ramp-to-freeway weaving vehicle must make to complete the ramp-to-freeway movement successfully.
- LCFR = minimum number of lane changes that a freeway-to-ramp weaving vehicle must make to complete the freeway-to-ramp movement successfully.
- NWL = number of lanes from which a weaving maneuver may be completed with one lane change or no lane changes.
How do option lanes affect the lane-changing inputs?
When an option lane exists, the freeway-to-ramp maneuver at the diverge point can be made without a lane change. If this principle is not observed, the lane-changing inputs will be coded incorrectly, resulting in inaccurate analyses for the entire segment. The figure below illustrates the cases of an incorrect modeling (when the exit maneuver is counted as a lane change) and the correct modeling approach (the exit maneuver is not counted as a lane change):
