Highway Engineering

For non-ideal conditions, Equation 4.17 becomes the following:

(4.18)

When lane widths are narrower than 3.65 m and/or barriers, lighting posts or

any such obstructions are closer than 1.83 m from the edge of the travelled pave-

ment (either at the kerb or median), an adjustment factor fwmust be introduced.

If the lane width is reduced to 2.74 m (9 ft) and there are obstructions at both

edges bounding it, the capacity will be reduced by 34%, or just over one-third.

Table 4.3 gives the adjustment factors for a 4-lane divided multi-lane highway.

Figures can also be obtained from the Highway Capacity Manual for 2-lane

undivided, 4-lane undivided and 6-lane divided and undivided highways.

Heavy vehicles such as trucks, buses and recreational vehicles have a negative

effect on the capacity of a highway due to their physical size together with

their relatively slow acceleration and braking. The resulting reduction in capac-

ity, termed the fHVcorrection, is estimated on the basis of the amount of road

space taken up by each of these vehicle types relative to that taken up by a

private car combined with the percentage of such vehicles in the traffic stream

in question.

SFij

i

C whvpE

v

c

()=¥ÊË ˆ ̄ ¥¥ ¥ ¥ ¥Nf f f f

82 Highway Engineering

Example 4.2 Contd

Solution

The service flow can be calculated knowing the hourly volume during the

peak hour and the peak hour factor:

SF=V∏PHF

= 1850 ∏0.8 =2312.5 vehicles per hour

C 70 =2000 passenger cars per hour per lane

N(the number of lanes in each direction) = 2

Since

Therefore

Under the prevailing ideal conditions, therefore, with reference to Table 4.2,

the ratio of flow to capacity is greater than 0.54 but less than 0.71. The

highway thus provides level of service C.

v

c

C

i

ËÊ ˆ ̄ =∏=SF j 2312 5..∏ ¥()2000 2=0 58

SFMax i j

i

C

v

() c

=¥ÊË ˆ ̄