Page 50 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

In South Africa, a general decrease in the number of short heavy vehicles has been observed from the late 1980s to
2003, while the number of long heavy vehicles has increased (De Bruin and Jordaan, 2004).

Provision must be made for long-term changes in traffic volumes and vehicle loading by:
• Traffic volumes: Apply a growth rate to the heavy vehicle traffic. Road geometric capacity restrictions are
essential to contain volume growth to realistic levels.
• Loading per vehicle: Apply an E80 per heavy vehicle growth rate in addition to the volume growth rate. Care
should, however, be exercised not to grow E80/HV to practically impossible levels. Guidelines on the E80/HV
growth rates are given in Table 18.

Adjustments for long-term variations are complex. Care should be taken to not overestimate the long-term changes
in the traffic volume and vehicle loading based on cyclic fluctuations lasting several years. These are usually related
to economic activity of the country. It is not advisable to correct for these cyclic variations as their average impact
corresponds with the trend. Consequently, any adjustment factors should be developed over at least a five year
period.

4.4.1 Lane Distribution
If traffic data is collected and processed on a lane by lane basis, a lane distribution is not required for multilane
facilities. However, if the ADE is calculated from traffic counts, or from the results of transportation planning models,
a lane distribution is required. The distribution between lanes may vary along the length of a road depending on
geometric design, climbing lanes and ramps. The distribution of traffic volume and loading is also normally not the
same. Suggested lane distribution factors are given in Table 13. The design ADE for each lane is calculated by
multiplying the total ADE per direction by the lane distribution factor, often termed B e .

For surfaced fast shoulders, the same pavement design is used as the adjacent lane.

Table 13. Recommended E80 (ADE) Lane Distribution Factors (B e )

Total Number of E80 Lane Distribution Factor
1
Lanes in Both Surfaced Slow Lane 1 Lane 2 Lane 3
Directions Shoulder
2 1.00 1.00 – –
4 0.95 0.95 0.30 –
6 0.70 0.70 0.60 0.25
Notes
1. Lane 1 is the outer or slow lane
2. For dual-carriageway roads

4.4.2 Adjustment for Short-Term Variation

If a traffic survey is conducted over a long enough period during a year, the influence of weekends, public holidays
and peak periods are averaged out, and the ADE provides a direct indication of the AADE. However, if the ADE is
determined from a relatively short survey, then the influence of night time, weekends and unusual periods is large,
and adjustments are needed for the following:
• Variation between day and night traffic, where data was only collected for a 12 hour period.
• Variation between weekday and weekend traffic.
• Variation between periods of exceptionally low traffic, such as holidays when businesses, factories and the
construction industry are closed, and periods of normal traffic.
• Variation between periods of exceptionally high traffic, such as harvesting season in agricultural areas, and
out-of-season periods.

The ADE is corrected with adjustment factors. Where applicable, the adjustment is done by calculating weighted
average values for the AADE, considering the duration of the normal and exceptional traffic in a year. The
adjustments must only be applied if:
• Variations occur year after year on the specific road.
• Traffic survey results do not represent the conditions for a full year.
• The adjustments have a significant impact, resulting in serious error if the adjustment is not applied.

Section 4: Design Traffic Estimation
Page 39