Cycle-by-cycle analysis of congested flow at signalized intersections
- Univ. of Illinois, Chicago, IL (US)
The increasing number of traffic signals and traffic signal systems in urban areas in the United States is influencing traffic-flow patterns on roadways. Because of the closer proximity of traffic signals (whether coordinated or not), there are fewer observed variations in traffic flow demand during peak hour, and fewer signals in urban areas now operate in truly isolated mode. This is especially true in the case of heavy demand volumes where upstream signals tend to filter traffic at the signal capacity (that is, at virtually a fixed rate). From the traffic engineer's standpoint, the issues associated with congested flow are quite different from those occurring under normal operation (some would even argue that congestion of gridlock is now considered normal operation). The following questions may be asked: How long will it take to clear the congestion developed during peak period; What maximum overflow queue can be expected; How much improvement can be achieved through better signal coordination The principal tool available to traffic engineers for the analysis of signalized intersections in the United States is the {ital Highway Capacity Manual} (HCM). Yet, none of the issues mentioned above is adequately addressed in the HCM. For example, the HCM method uses a fixed peak period of analysis of 15 minutes for calculating level of service. The HCM method has no mechanism for estimating queues, or for estimating when peak-period queues can be cleared on an approach. Furthermore, signal coordination effects are expressed by a set of progression factors that have a very loose correspondence to signal offsets. More important, all analyses in the 1985 HCM are performed assuming average conditions in 15-minute peak period without considering the cycle-by-cycle variation in flows, queues, and delays. Finally, the level-of-service analysis is limited to situations with no initial queue at the start of the peak period and to volume-to-capacity ratios under 1.20.
- OSTI ID:
- 5474982
- Journal Information:
- ITE (Institute of Transportation Engineers) Journal; (United States), Journal Name: ITE (Institute of Transportation Engineers) Journal; (United States) Vol. 61:3; ISSN 0162-8178; ISSN ITEJD
- Country of Publication:
- United States
- Language:
- English
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