EVALUATING CAPACITIES OF ONE-LANE ROADS WITH TURNOUTS

Speed-flow relationship models for one-lane roads with two-way traffic are developed. Each model considers a composite variable of speed divided by the traffic distribution ratio as a dependent variable and both traffic distribution ratio and volume as independent variables. The traffic distribution ratio represents the degree of traffic conflict and is measured as the percentage of one-way traffic on the heavy-traffic direction to total traffic. A 1982 traffic survey of four study sites in the Mount St. Helens Monument region forms the data base. The following are specific findings of the study: (a) Model specification and coefficients, including elasticities, are stable. (b) The capacity of a single-lane road with turnouts may exceed 400 vehicles per day without reaching the congested-flowing situation when the majority of traffic is controlled by citizen band radios. (c) Speed is more sensitive to traffic distribution than to volume. (d) The predictive ability of the developed models has been validated at nine study sites with satisfactory results. The results of this study provide road engineers and managers some guidelines for selecting the most cost-effective design standard and management strategy for one-lane roads with turnouts.