Page 49 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

If the survey period is fully representative of the traffic conditions for the full year, then the ADE equals the AADE.
However, if the survey period is not representative of the traffic conditions for the full year, the ADE needs to be
carefully converted to the AADE. Figure 22 illustrates two different ADEs determined from different surveys on the
same route. In situations like this, the ADE needs to be adjusted to account for the variations, as discussed in
Section 4.4.
ADE from Survey A

Number of Daily E80s per Lane Survey ADE from Survey B

Traffic

A

True AADE
Traffic
Survey
B

One Year, or 365 Day Period

Figure 22. Potential Deviation of the ADE from the AADE

4.4 Traffic Loading Variation

Variations in traffic loading occur over the short or long-term:
• Short-term variations occur within a year. Seasonal variations are
recurring periodic patterns occurring each year, and are usually related to Traffic Surveys
weather or agricultural activities, such as planting and harvesting. Periods
of exceptionally low traffic activity may also occur when economic activity Great care must be taken to
is halted at a specific time of the year. Corrections for seasonal variation ensure that traffic during the
and exceptional periods are made using adjustment factors, but should survey period is representative
only be applied if they are expected to occur each year for the duration of of the traffic during the year.
the design life of the facility.
• Long-term variations include changes in traffic volumes and loading of vehicles over a period longer than a
year. Traffic volume changes are affected by a host of factors, for example, population growth or decline,
establishment of new industries such as mines and quarries, long-term construction programmes and economic
recessions. Traffic volumes and vehicle loading are also influenced by changes in the way the preference shifts
between different transportation modes, particularly road, rail and pipeline.

Three main factors influence the equivalent standard axles per heavy vehicle:
• Improved vehicle design leads to changes in the type and size of heavy vehicles.
• Efficient management of road transport, i.e., fewer vehicles running empty and better utilisation of vehicle
capacity.
• Level of overloading control.

Changes in Government regulations and policies, such as changes in the legal
axle load or gross vehicle mass of vehicles, and deregulation of road transport Adjustments for Long-
have a significant effect on traffic loading. The degree to which law Term Variations
enforcement is applied to overloading also has a significant effect on the Adjustments for long-term
loading per vehicle. variations are complex.
Adjustment factors should be
developed over at least a five
year period.

Section 4: Design Traffic Estimation
Page 38