Arrival Turning Flows
Average Daily Traffic
Entry Width E
Number of Entry Lanes ne
Entry Radius R
Entry Angle Phi (Ф)
Flare Length L’
Flare length is the distance from the entry to the halfway point in the approach.
HCM does not use the flare length L'.
When V > E then L' = 0.
When E is greater than V, increasing L' will increase capacity.
The increase in width from V to E should be at a uniform rate.
The increase in capacity is proportional to the area (E-V) multiplied by L'/2.
Consequently if E-V is very small, even a large L' will produce only a small increase in capacity.
Even widening V to match the slightly larger E only increases capacity a little.
When E is significantly greater than V, increasing L' produces large increases in capacity that level out at about 300 ft.
With L' = ~312 ft (100 m) the capacity is 95% of the capacity achieved by widening V to match E.
If there is a parallel approach E wide for a distance d back from the yield line followed by a taper back to the approach width V then L' is measured from the back of the parallel section to the width (E+V)/2 as normal. However, L' is then increased by adding d.
This is a good way of increasing capacity when a long L' from the yield cannot be achieved due to ROW constraints as d + a short L' can give the same capacity increase a longer L' with no parallel section. All the above depends on both E and V being effective. L' should also be effective with no parked cars etc.
© Rodel Software Ltd