Evaluation of Outside and Inside Expressway Ramps Based on Merging Probability

Japanese urban expressway networks contain not only the usual outside ramps, but also many inside ramps. Although the inside ramps do not block off adjacent and always‐expensive urban property, the design is less desirable operationally than one with outside ramps. This report describes a mathematical method to evaluate which of the two on‐ramp designs is better in terms of merging probability. Knowing the probability allows us to evaluate the operating conditions and capacities at merging area. A series of traffic surveys are conducted to accumulate data on the traffic‐flow characteristics of outside and inside ramps on the Hanshin Expressway network in Kobe, Japan. The headway distribution on the through lane and the acceptable gaps of the merging vehicles were measured from video tapes. The merging probabilities are calculated using those survey results. The results indicate higher merging probability for the outer parallel type. It is suggested that the inside‐ramp merging‐lane length should be longer than that of the outside ramps and that attention should be given to operational countermeasures such as speed regulations.