Two-lane highways have one lane for the use of traffic in each direction. The principal characteristic that separates two-lane highway operation from that of other uninterrupted-flow facilities is that passing maneuvers take place in the opposing lane of traffic. Passing maneuvers are limited by the availability of gaps in the opposing traffic stream and by the availability of sufficient sight distance for a driver to discern the approach of an opposing vehicle safely. As demand flows and geometric restrictions increase, opportunities to pass decrease. This creates platoons within the traffic stream, with trailing vehicles subject to additional delay because of the inability to pass the lead vehicles.
Because passing capacity decreases as passing demand increases, two-lane highways exhibit a unique characteristic: operating quality often decreases precipitously as demand flow increases, and operations can become “unacceptable” at relatively low volume-to-capacity ratios. For this reason, few two-lane highways ever operate at flow rates approaching capacity; in most cases, poor operating quality leads to improvements or reconstruction long before capacity demand is reached.
Source: 2010 Highway Capacity Manual, Volume 2: Uninterrupted Flow, Chapter 15: Two-Lane Highways The remainder of this chapter provides instructions and examples for performaning highway capacity and level-of-service analyses for this type of facility. Drafts of new chapters of the HCM for the 6th Editions are available for a short time on the Volume 4 web page.