Page 58 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

Detailed project specific traffic surveys are not necessary for Category C and D projects, and replaced with a visual
observation of the traffic and vehicle loading characteristics. For Categories A and B, it is recommended that either a
7 day count with limited static weighing, or a full 7 day axle load survey is done. Figure 25, from the SADC Low-
volume Sealed Roads Guideline, shows the effect of the count duration on the error in the traffic estimates.

Figure 25. Effect of Counting Duration on Accuracy of Results

A particularly difficult problem is the design of a green fields project, where traffic data cannot be collected. In such
cases, only published results or transportation planning models can be used. The preferred option is a transportation
planning model, done by a transportation engineer.

4.6.1 Published Results
When using published results, the traffic conditions must be the same. To check this, compare the loading per heavy
vehicle, the type/classes of heavy vehicles operating on the road, or using a stratification system.

When published results or those measured on other roads are used, TRH16 warns that significant errors in the
design traffic estimate can result. This technique should therefore only be applied to Category C and D roads
carrying little traffic (less than ES0.3). In such situations, a basic traffic count significantly enhances the design
traffic estimation.

A stratification system classifies routes based on the function of the route, in addition to the characteristic of the
heavy vehicles. A simple system considers the proportion of 2-axle heavy vehicles in the total heavy vehicle traffic
spectrum as an indicator of the type of route (Bosman, 1989 and 2004). TRH16 lists the E80/HV based on the
original work by Bosman. However, given the influence of the 1996 increase in legal axle load limits and the
influence of the level of overloading control, those E80/HV values
may be too low. It is therefore recommended that the E80/HV
values listed in Table 18 for the short-medium-long heavy vehicle
classification, considering the level of overloading control and the TRH16 E80/HV Values
proportion of short heavy vehicles, are used. Because of the The TRH16 (1991) recommended values
general lack of E80/HV growth data it is recommended that the for the E80/HV and the growth rate were
growth rates in Table 18 are also used. In general, TRH16 is determined before the 1996 increase in
outdated, and needs updating. the legal axle load, and do not consider

the level of overloading control.
It is therefore recommended that the
values given in Table 18 are used for
E80/HV and the growth rates.

Section 4: Design Traffic Estimation
Page 47