Geometry > Entry Geometry > Flare Length L'

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.

The Capacity / Flare length graph above is with V = 13 ft (4 m) and E = 26 ft (8 m) so E-V is 13 ft (4 m). Increasing L' produces a sharp capacity increase that starts to level off after about 230 ft (70 m).

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.

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