Traffic Volume
The role of the traffic engineer is to enable all traffic to travel on the road at a reasonable speed and with an appropriate degree of safety. This is not the loading that is used in the Pavement Design. These values are used to determine the road width only.
With relation to the volume of traffic using the road, the passenger car is adopted as the standard unit and other vehicles are assessed in terms of passenger car units (pcu).
Differences in the urban and rural situations arise due to the variation of speeds in the two areas. Decisions on road width are not normally made on total traffic flow per day as this is misleading but rather on the peak hourly flow. In Britain the maximum permissible flow is 3,000 pcu/h for a two lane dual carriageway and 4,500 pcu/h for a three lane dual carriageway (motorway). For all purpose roads with junctions these figures reduce to 1,100 ‘‘and 1,900 pcu/h respectively.
Where the road is new, studies must be carried out to estimate the volume of traffic expected to use the road. Where the new road replaces existing roads this is not too difficult. If however the road is expected to change the flow of traffic then analysis should be carried out as to the volume and constitution of traffic on the new road. Matters are further complicated if the road is very long or provides access to or from a large town. Computer methods are now available to aid in this process. The constitution of traffic on the new road is of interest to the pavement engineer.
Traffic Loading
In this section, we will discuss the traffic loading that is taken forward to the pavement design section. Unlike in the above section where Passenger Car Units were the reference unit, we will work in Standard Axles. This is a reference unit to determine the average loading on a pavement by what is known as the standard axle. This then allows a total loading over the life of the pavement to be determined, normally in Millions of Standard Axles (msa).