Highway Engineering

Where the road gradient is greater than 3% over a distance of^1 / 2 mile or less

than 3% but over 2% over a distance greater than 1 mile, these values are no

longer valid and more detailed tables as presented in the Highway Capacity

Manual (TRB, 1985) must be utilised.

Having obtained the necessary pce valuations, the overall correction factor

can be estimated once the percentages of the three vehicle types present along

the section of road in question has been arrived at:

PT– Percentage of trucks in traffic stream

PB– Percentage of buses in traffic stream

PR– Percentage of recreational vehicles in traffic stream.

Given these values, the correction factor,fHV, can be derived as follows:

(4.19)

If the driver population is deemed not to be ideal, i.e. not composed entirely

of regular weekday commuters, then a reduction factor can be utilised, reduc-

ing the capacity of the highway by anything between 10% and 25%. There are

no quantitatively derived guidelines that can assist in making this assessment.

Professional judgement must be the basis for the valuation used. The value range

is illustrated in Table 4.5.

With regard to the type of highway, the ideal situation is represented by a

divided highway in a rural setting. If, however, the highway is undivided and/or

the setting is urban based, a correction factor must be used to take account of

the resulting reduction in capacity. This correction factor,fE,reflects the reduc-

tion in capacity resulting from the absence of a physical barrier along the centre-

line of the road, with consequent interference from oncoming traffic together

with the greater likelihood of interruptions in the traffic stream in an urban or

suburban environment.

Values offEgiven in the Highway Capacity Manual are shown in Table

4.6.

f

HV PETT PEBB PERR

=

+-{}()+-()+-()

1

1 111

84 Highway Engineering

Driver classification Correction factor,fP

Regular weekday 1.0

commuters

Other classes of drivers 0.9–0.75

Table 4.5Correction factors for driver population
types (Source:Highway Capacity Manual(TRB,
1985))

Highway classification Divided Undivided

Rural 1.0 0.95

Urban/suburban 0.9 0.80

Table 4.6Correction factors for highway environment

(Source:Highway Capacity Manual(TRB, 1985))