PLANNING AND DESIGN GUIDELINES FOR THE DEVELOPMENT OF A SYSTEM OF PASSING LANES FOR ALBERTA HIGHWAYS

This paper presents the planning and design approach developed by alberta transportation for the provision of passing lanes on rural two lane highways. Determination of the need for passing lanes is based on a comparison of the demand for overtaking with the supply of overtaking opportunities. The unified model, so called because it unifies the supply and demand functions, embodies the concept that drivers perceive level of service on their demand for and opportunity to overtake slower moving vehicles. The demand function is determined from speed distributions or catch up rates while the supply side is a function of road geometry and the availability of gaps, adequate for safe overtaking, in the opposing traffic stream. Passing lane candidate highway sections are identified by rank ordering on the basis of net passing opportunities and unsatisfied passing demand as determined by the unified model. Although there are many similarities between passing lane design guidelines in alberta and Australia, the question of permitting passing in the opposing lane and the development of an appropriate signing system has been a particular concern in Canada. Research needs are discussed in the light of findings observed during the course of using the unified model and monitoring of passing lanes (a).