The derivation is described of volume warrants and design charts for left-turn storage lanes at unsignalised grade intersections on four-lane and two-lane highways. The design charts are based on a theoretical analysis and on a series of field studies of traffic behaviour at intersections. The analysis is based on a queuing model in which arrival and service times are assumed to follow a negative exponential distribution. The arrival rates are determined by the volumes of left-turning, through or advancing and opposing traffic, and by the time interval required by the vehicle to clear the advancing lane. Service rates are determined by volume of opposing traffic, and by the time interval required to make a left turn manoeuvre. A left-turn storage lane is warranted by traffic volumes when the ratio of arrival rate to service rate exceeds a specified value, which varies with type of highway and vehicle speed. The required storage length of the lane may also be determined. Field studies were conducted of traffic behaviour at seven unsignalised Ontario intersections. Questionnaires on suitability or chart recommendations filled out by highway department regional traffic engineers at 80 Ontario intersections indicated general agreement.
Abstract