Some Observations Of Highway Traffic In Long Queues

The arrival times of vehicles traveling southbound along a two-lane, bi-directional highway were recorded at eight neighboring locations upstream of a bottleneck caused by an oversaturated traffic signal. Cumulative curves constructed from these observations describe completely and in great detail the evolution of the resulting long queues. These queues formed directly upstream of the signal when the signal's service rate fell below the southbound arrival rates, and never formed away from the bottleneck. The predictability of bottlenecks like the one studied here can be exploited to manage traffic more effectively. The behavior of vehicles within the queue, however, was rather interesting. While the flow oscillations generated by the traffic signal were damped-out within one-half mile of the bottleneck, it was found that other oscillations arose within the queue farther upstream, at varied locations, and then grew in amplitude as they propagated in the upstream direction. Thus, the queue appeared to be stable close to the bottleneck and unstable far away. Oscillations never propagated beyond the upstream end of the queue, however; i.e., the unusual phenomena always arose after the onset of queuing and remained confined within the queue. Some of these findings run contrary to current theories of traffic flow. As the data set collected in this study is unprecedented in scope and detail, and so that it may be of use to other researchers, it has been posted on the internet and is fully described here. KEY WORDS Bottleneck Observation; Car Following; Jam Development; Queueing; Traffic Instability