Defining Freeway Capacity as Function of Breakdown Probability

The need for an enhanced freeway capacity definition that incorporates the probabilistic nature of the freeway breakdown process is addressed. This is investigated through an extensive analysis of speed and volume data collected at two freeway bottleneck sites in Toronto, Ontario, Canada. At each site, the freeway breakdown process was examined in detail for more than 40 congestion events occurring during the course of nearly 20 days. Preliminary models were developed for each site. These models are used to describe the probability of breakdown versus the observed flow rate and to examine the implications that this probabilistic approach to breakdown has on the current definition of freeway capacity. A revised, probabilistic freeway capacity definition is proposed for use in future editions of the Highway Capacity Manual.