Two-Lane Highway Passing

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.

 Components and values of passing sight distance (AASHTO)

Passing in oncoming lane The most distinguishing feature of traffic operations on two-lane highways is passing in the oncoming lane (when passing lanes are not present). Therefore, this passing maneuver is constrained by not only the amount of opposing-lane distance used in the execution of a passing maneuver, but also the sight distance and clear-distance (or gap size) a follower requires before attempting a passing maneuver. The former issue depends on road design and markings of no-passing zone, while the latter issue depends on traffic demands. The following subsections describe the various components of logic employed to determine when and how a vehicle will perform a passing maneuver in the oncoming lane.

When will a vehicle attempt to pass a vehicle in front of it?

1. Determine if the subject vehicle is in a following mode Currently, the program defines a vehicle as being in a following mode when the time headway between it and the vehicle immediately in front of it is equal to or less than or equal to 2.5 seconds—this is currently the logic the HCM uses to approximate % Followers. The value of 2.5 seconds, however, can be changed by the analyst in the settings. Additionally, the trailing vehicle must be traveling at a speed at least equal to the speed of the leading vehicle. If the subject vehicle is determined to be in a following mode, then the following steps are carried out to determine if the following vehicle will attempt a passing maneuver.

2. Determine tolerable speed If it is determined that a vehicle is in a following mode, then the tolerable speed for that vehicle is calculated. Tolerable speed is defined as the maximum speed at which the desire to pass for a following driver will be 100 percent. Tolerable speed varies for different driver types (See Eq. 12 for the computation), due to the different degree of aggressiveness for each driver type. Note that a driver’s desired speed is a function of free-flow speed and driver type. For example, a driver of type 1 will have a desired speed of 88% of the link free-flow speed, while a driver of type 10 will have a desired speed of 112% of the link free-flow speed (these percentages of free-flow speed can be modified by users on RT 147 of the TRF input file).

Desire to Pass (Oncoming Lane)

Acceleration Values (Oncoming Lane)

Acceleration Values (Passing Lane)