Highway Engineering

4.5 The UK approach for rural roads,

4.5.1 Introduction

In the past, predicted flows in the fifteenth year after opening were used as a

reference for the selection of a carriageway standard most likely to be opera-

tionally acceptable (DoT, 1985). The most recent advice note (DoT, 1997a) now

advocates using opening year flows as a starting point for assessing new rural

trunk road links.

Under the British system, the process for the selection of carriageway stan-

dard can be summarised as:

 Determine the 24-hour annual average daily traffic (AADT) estimate (high

and low growth values) for the opening year of the scheme. These estimates

must allow for the effects of induced traffic levels.


 Compare the quantities of flow derived with the ranges of flow specified for

a number of carriageway standards (see Table 4.12).


 Choose those carriageway standards within those value ranges where either

or both of the above flow predictions lie. For example, forecasted AADT

values within the range of 25 000 to 27 000 show that D2AP and D3AP car-

riageway standards are suitable for assessment.


 Having taken account of any potentially important local factors such as the

cost of construction and maintenance, network effects or severe impacts on

the environment, both economic and environmental assessments can be

carried out in order to select the optimal standard.

The process is substantially at variance with the ‘level of service’ approach.

Table 4.12 illustrates the opening day AADT ranges for the different car-

riageway widths and road types for rural highways.

4.5.2 Estimation of AADT for a rural road in its year of opening

Selection of the appropriate layout for a rural road requires that the high and

low forecast for the opening year is estimated. This is undertaken using the

92 Highway Engineering

Example 4.6 Contd

From Table 4.2, the maximum ratio of flow to capacity for LOS C is 0.71.

The number of lanes required can thus be estimated as:

Therefore, rounding up, three lanes are required in each direction.

N=∏ 3200 ()2000 0 71 1 0 1 0 1 0 1 0¥. ¥¥¥¥.... .=2 25