Passing lanes (also known as three-lane highways), wide paved shoulders, and extra-wide highways are designed to enhance traffic operations on two-lane highways. These low-cost improvements bridge the gap between conventional two-lane highways and four-lane highways. This paper compares design practices and policies as well as the operational experience regarding passing lanes, wide paved shoulders, and extra-wide two-lane highways. Canadian and German warrants for upgrading a two-lane highway to a passing lane system and ultimately a four-lane motorway are examined. Geometric design standards for cross-section elements such as lane width and shoulder width are compared. Design standards for the length and spacing of passing lanes as well as the length of merge and diverge tapers are examined. Design practices and policies concerning transition areas between, for example, conventional two-lane and extra-wide highways are discussed. The signing and marking practices for passing lanes, extra-wide highways and transition areas are also included in the paper. The paper discusses the operational experience in terms of overtaking rates, speed-volume relationships, platooning, level-of-service, accident rates, and driver attitudes. Research studies in both Canada and Germany show that passing lanes, wide-paved shoulders, and extra-wide highways increase the quality of service and decrease the accident rates compared to conventional two-lane highways. Both Canadian and German studies have concluded that passing lanes and extra-wide highways offer a cost-effective alternative of enhancing the quality of service and safety on those two-lane highways that do not meet the warrants for motorways or where cost and environmental concerns rule out the four lane option.
Samenvatting