Arrival Turning Flows
Average Daily Traffic
Circulating Lanes nc
Circulating Width C
The circulating width only affects capacity as it defines the number of circulating streams so neither the HCM nor the geometric capacity models use C for capacity estimation directly. C only affects capacity if it changes the number of circulating traffic streams.
If C allows two circulating streams, increasing C does not increase capacity until C is wide enough to accommodate three circulating streams.
The capacity of the Leg 1 entry is affected by the number of opposing circulating streams at C. For a fixed circulating flow past an entry, the entry capacity increases as C is widened from 1 to 2 to 3 to 4 traffic streams. At low circulating flows the capacity increase is very small, but is larger the greater the circulating flows. The figure below shows the capacity for 1, 2, 3 and 4 opposing streams.
Leg 1 entry capacity can also be affected by the number of receiving circulating lanes at C+.
The receiving circulating width C+ must accommodate the traffic streams feeding it from the Leg 1, otherwise the entry will be forced to reduce the number of feed streams greatly reducing entry capacity. Sometimes the number of circulating lanes needed is determined by the number of traffic streams from other entries.
Altering the circulating width to increase the number of streams can require serious modification of other geometric parameters. Leaving C+ alone and changing others may be less disruptive. However, C+ must allow lane consistency.
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