MEASURING LEVEL OF SERVICE OF TWO-LANE HIGHWAYS BY OVERTAKINGS

A level-of-service concept that is based on the supply of passing opportunities and demand for overtaking is presented. A driver is hypothesized to perceive level of service on a two-lane highway on the basis of his or her ability to overtake slower vehicles. The demand for overtaking is a function of volume and the speed distribution characteristics of the traffic stream. The supply of opportunities for vehicles to overtake is a function of the number of gaps adequate for safe overtaking maneuvers in the opposing traffic stream and the percentage of passing zones of the highway section under consideration. The relationship between supply and demand for overtaking forms the basis of a level-of-service measure defined by the overtaking ratio. The overtaking ratio is defined as the ratio of the achieved number of overtakings on a two-lane highway to the desired number (or to the total number of overtakings possible on a two-lane highway with continuous passing lanes and with vertical and horizontal geometry similar to the two-lane highway). Various level-of-service measures and procedures including the method of the 1965 and 1985 Highway Capacity Manuals, the percent-following count generated by simulation modeling, and the overtaking ratio, are compared. The overtaking ratio decreased much faster than the percentage of time delayed increased for those ranges of level of service to which motorists are most sensitive on two-lane highways. The overtaking ratio is suggested as another dimension of level of service to be considered for two-lane highways in addition to existing measures such as percentage of time delayed, capacity use, and speed.