Passing lane segments can significantly improve traffic flow, especially on steep and long grades. However, the merging behavior of vehicles at the end of the passing lane can become problematic at high flow rates. Higher flow rates reduce the average gap between vehicles, which forces drivers to merge into smaller gaps. This behavior creates shockwaves, as following vehicles must decelerate for the merging vehicles. At some point, breakdown is reached, and the performance of the passing lane degrades below that of a nonpassing lane segment. Maximum flow rates for passing lane segments are a function of the heavy vehicle percentage and the vertical classification. For example, the capacity flow rate at the merge point on a vertical class 5 passing lane segment with 25% or higher trucks is approximately 1100 veh/h.
Level of Service If the demand-to-capacity ratio is less than or equal to 1.0, follower density is used as the service measure for all two-lane highways. However, two sets of LOS thresholds are used to account for differences in driver perception between driving on higher-speed versus lower-speed highways.