Highway Engineering

4.4.5 Sizing a road using the Highway Capacity Manual approach

When sizing a new roadway, a desired level of service is chosen by the designer.

This value is then used in conjunction with a design traffic volume in order to

select an appropriate cross-section for the highway.

Within the US it is common practice to use a peak hour between the tenth and

fiftieth highest volume hour of the year, with the thirtieth highest the most widely

used. In order to derive this value for a highway, the annual average daily traffic

(AADT) for the highway is multiplied by a term called the Kfactor, such that:

(4.23)

where the value ofKicorresponds to the ith highest annual hourly volume

Typical values for a roadway are:

K 1 (the value corresponding to the highest hourly volume) =0.15

K 30 (the value corresponding to the thirtieth highest hourly volume) =0.12

If theK 1 value is used, the road will never operate at greater than capacity but will

have substantial periods when it operates at well under capacity, thus represent-

ing to some extent a waste of the resources spent constructing it. It is therefore

more usual to use the K 30 figure. While it implies that the road will be over capac-

ity 29 hours per year, it constitutes a better utilisation of economic resources.

Design hourly volume DHV()=¥Ki AADT

90 Highway Engineering

Example 4.5 Contd

If we assume that level of service E is provided in this case, the correctional

factors for both lane width and heavy vehicles can be estimated:

fw=0.97 (3.65 m wide lanes, 1.22 m (4 ft) clearance on hard shoulders)

Level terrain, level of service E, therefore ET=2.0,EB=1.6 and ER=1.6

Therefore:

In order to calculate the actual level of service:

Therefore the assumption of level of service E was a correct one and con-

sistent with the correction factors computed.

v

c

=∏¥¥¥()fffdwHV

=∏()¥¥¥

=

Service flow 2800

1500 2800 0 94 0 97 0 88

065

...

.

fHV=

+{}()+ ()+ ()

(^1) =
101010 04 0 6 00206

088

.....

.