The Effect of Small Cars on the Capacity of Signalized Urban Intersections

Time headways between vehicles being discharged from a queue at a signalized urban intersection is a measure of the intersection’s capacity. An event recorder actuated by tape switches on the road surface is used to measure headways at various signalized intersections in Toronto. The headways during saturation flow are related to the size of the vehicles and it is found that vehicles follow a small car with a closer headway than a full-sized car and a small car follows a vehicle closer than a full-sized car. The combined effect when both the lead and following vehicles are small cars is found to yield the smallest headways. This combined effect is most significant for the vehicles at the beginning of the queue. As a result, it is estimated that the capacity of a signalized intersection is increased by up to 15% for a stream of small cars over a stream of full-sized cars.