Arrival Turning Flows
Average Daily Traffic
Graphical Geometry Editor
Grade Separation G
Number of Approach Lanes nv
Approach Half Width V
This is the narrowest width of the approach road prior to any widening (flaring) up to the roundabout entry.
The width excludes gutter pans.
The geometric model has a range from 6.5 ft (2 m) to 40 ft (12 m)
When there is a bypass lane then V may be modified and need to be determined together with the bypass Approach Width Vb and the Total Approach width Vt.
HCM does not use V, but the Rodel uses V in HCM mode to check and warn if the approach road has sufficient capacity to accommodate the arrival flows.
Geometric Capacity is very sensitive to V so the presence of bus stops or parked cars are a serious consideration and only the “effective” value (that which is actually used by approaching traffic) should be used as input, otherwise capacity may be significantly overestimated.
The graph below shows the variation in capacity with V (for with fixed circulating flows) at a three lane entry.
V is a given rather than a variable the designer can change. However, in some circumstances V can be increased by offsetting the center line of the road provided the narrower exit road is acceptable.
V is a parameter in the accident models.
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