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Freeway capacity, saturation flow and the car following behavioural algorithm of the VISSIM microsimulation software

Freeway capacity, saturation flow and the car following behavioural algorithm of the VISSIM microsimulation software
Julian Laufer Maunsell Australia, Sydney, NSW, Australia



Freeways represent the best example of an unconstrained traffic flow system with high speed operations, limited access and egress opportunities and few factors to inhibit traffic flow. This road type can typically achieve the highest throughput of traffic per hour per lane across most metropolitan networks. As most regular limitations to free flow traffic conditions are removed from freeways, the achievable throughput is subject to regulatory constraints such as speeds and societal limitations such as headways between vehicles. Historical traffic volumes indicate that hourly throughputs per lane are generally increasing over time. This reflects a reduction in headways between vehicles as motorists’ acceptable and safe gaps between vehicles become smaller. As a direct consequence, the capacity of freeways has been observed to increase over time. The most recent edition of the US Highway Capacity Manual indicates a ceiling value significantly higher than that applied fifteen years prior. This paper has set out to identify the changing values of capacities over time as applied to freeway traffic conditions. The paper examines capacity in the context of observed volumes to date from both an Australian and International experience. This paper then examines the car following algorithm in the VISSIM microsimulation software to benchmark the similarity of the maximum freeway capacity against these observed volumes. This review of existing literature was initially undertaken when developing a VISSIM microsimulation model to examine the application of forecast…...

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