5.2.3 Distribution of Traffic
On multi-lane roads, the distribution of vehicles over the available lanes must be determined. Procedures for this are described in Section 5.2.4.
The traffic volumes used throughout the seal design procedures are based on the general mix of light and heavy vehicles, with the heavy vehicle proportion assumed to be between 5 and 10% of the total. It is important to determine the actual percentage of heavy vehicles.
In some locations, the proportion of heavy vehicles in each lane may not be uniform or may not be equal in both directions because of restrictions, specified routes, specified lanes such as bus lanes, overtaking/climbing lanes, or roads with a third lane in the centre used as passing lanes by traffic in both directions.
On multi-lane roads, the percentage of heavy vehicles should be calculated separately for each lane, based on the mix of light and heavy vehicles estimated to use each lane. Worked examples have been included in Appendix C. Adopting an overall or average heavy vehicle percentage across all lanes given as part of AADT (e.g. 28%) will often result in an incorrect design traffic and/or traffic adjustment being applied.
Design consideration should be given to whether the heavy vehicles are evenly loaded/unloaded in both directions, or predominantly travel loaded/empty in one direction only. This is particularly important where for example the road may be an access to an industrial estate, a quarry access road, or a road connecting major industrial centres. Specialist advice should be sought for these situations.