A theoretical model for the permissive right-turn capacity of shared lanes with an opposed flow at traffic signals

This paper presents a theoretical model for the calculation of shared lane capacity (i.e., lanes with a mixture of straight ahead and right-turning vehicles) with an opposed flow at traffic signal controlled intersections. The model is a development of the Swedish Capacity Manual and the U.S. 1985 Highway Capacity Manual methods and assumes a binomial distribution of direction assignments in the subject lane, a random arrival pattern on the opposing approach and a step function for the gap acceptance requirement of right-turners. The model also accounts for any number of spaces available for right-turners to queue within the intersections without blocking the lane for following vehicles. Signal settings are assumed to operate on a fixed-time basis and results are presented to illustrate the effects of different green time settings and waiting spaces on shared lane capacity. The model was tested against field data collected at two such intersection entries in the Sheffield urban area. The calculated capacities were found to compare well with those observed in reality; statistical analysis indicaitng that the differences between observed and calculated values were not significant at the 95% confidence level. These results compare favourably with those obtained by other theoretical models, such as the 1985 HCM method and the TRRL RR67 procedure.