Little attention has been paid to the effects of motorcycles on the capacity of signalized intersections. They have so far been considered only from the viewpoint of traffic safety with respect to the design and operation of intersections. In South East Asian developing countries in, motorcycles are the most commonly used means of transport, making up a large part of the demand: for example, 25% in Bangkok, Thailand, and more than 50% in Hanoi, Vietnam. Motorcycles impede passenger car flow in two ways: increasing start-up lost time and decreasing the saturation flow rate. The influential factors associated with these inefficiencies, such as relative position to passenger cars and the number of rows formed by motorcycles lined up behind the stop line, were examined through statistical tests and regression models using traffic data measured at intersections in two capital cities in South East Asia, Hanoi in Vietnam and Bangkok in Thailand.
Therelative position of motorcycles to passenger cars significantly affected the mean headway. The saturation flow rate decreased at most 40% in accordance with the positional pattern. Start-up time also increased parabolically in proportion to the number of motorcycles in the Hanoi data and linearly in the Bangkok data. However, the characteristics of start-up lost time were too complex to describe it only by means of the number of rows.