Experimental investigation of spatial breakdown evolution on congested freeways

Abstract

Although urban and suburban freeways are designed to enable smooth flow with high speed, they experience traffic congestion during peak day periods especially when traffic demand is high but also during unexpected incidents. Traffic breakdown is identified when there is a rapid drop in speed level and a simultaneous steep rise in density level during the phase transition from free-flow to breakdown flow. The recovery from traffic breakdown is achieved after speed and density return to their pre-breakdown level in the uncongested conditions. This paper evaluates the spatial dependency of breakdown and investigates the consistency of time of breakdown between weekdays, based on data collected from a suburban section of Interstate 66 in Virginia. The identification of location and weekday effects is conducted by experimental design techniques. The spatial dependency of the transition times is evaluated through raw data in the speed–time plane and further examined by the shockwave linear propagation principle. Both approaches are generally consistent and produce similar results.