Highway Engineering

All these correction factors are as defined in the previous sections except for fd
which takes account of any unequal distribution of the traffic between the two
directions of flow.
The capacity of the flow in the peak direction decreases as the distribution
becomes more unequal. The value for different flow splits between 50/50 and
100/0 are given in Table 4.8.

Basic Elements of Highway Traffic Analysis 87

Distributional split 50/50 60/40 70/30 80/20 90/10 100/0

Correction factor,fd 1.0 0.94 0.89 0.83 0.75 0.71

Table 4.8Correction factor for different directional splits on a 2-lane highway
(Source:Highway Capacity Manual(TRB, 1985))

Therefore, if all the flow is in the peak direction, the capacity is effectively
71% of 2800 pcu/h, i.e. just under 2000 pcu/h.
The correction factor for lane widths/clearance on hard shoulders also varies
with the incident level of service. Typical values are given in Table 4.9.

Clearance on

hard shoulder

3.65 m lanes (12 ft) 2.75 m lanes (9 ft)

(metres) LOS A-D LOS E LOS A-D LOS E

1.83 or greater 1.00 1.00 0.70 0.76

1.22 0.92 0.97 0.65 0.74

0.61 0.81 0.93 0.57 0.70

Zero 0.70 0.88 0.49 0.66

Table 4.9Correction

factors for non-ideal

lane widths and

clearances from

obstructions (2-lane

highways) (Source:

Highway Capacity

Manual(TRB, 1985))

Intermediate values for 11 ft and 10 ft lane widths are available

The correction factor for heavy vehicles also depends on the incident level of
service. Typical values are given in Table 4.10.

Type of terrain

Vehicle type Level of service Level Rolling Mountainous

A 2.0 4.0 7.0

Trucks,ET B or C 2.2 5.0 10.0

D or E 2.0 5.0 12.0

A 1.8 3.0 5.7

Buses,EB B or C 2.0 3.4 6.0

D or E 1.6 2.9 6.5

A 2.2 3.2 5.0

RV ’s ,ER B or C 2.5 3.9 5.2

D or E 1.6 3.3 5.2

Table 4.10Passenger

car equivalents for

different classes of

heavy vehicles (2-lane

highways) (Source:

Werner and Morrall

(1976))