Page 59 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

4.6.1.1 Comprehensive Traffic Observations (CTO) Stations
SANRAL have installed a number of Comprehensive Traffic Observation (CTO) stations on the national road network.
Traffic is monitored for varying periods of time at these stations, but counts as long as one year on certain important
routes are not uncommon.

The information available from the CTO stations includes calculation of the average daily E80s for the particular
count. This information can be used to calculate the AADE. The information can also be used to calculate E80 and
E80/HV growth rates if historic counts from the same station are available. An example of the output obtained from
a station at Kinkelbos on the N2 in the Eastern Cape is shown in Figure 26.

4.6.2 Transportation Planning Models
Transportation planning models predict traffic patterns in terms of traffic distribution, growth, and future volumes.
Various inputs are used, such as economic activity and geographical zones. The models are usually based on an
urban modelling procedure, which looks at the region as a number of zones (van Zyl, 1986). Traffic is applied to the
zones and the number of trips generated and attracted to each zone is modelled. The trips are assigned to each link
of the network to obtain the traffic flows. This modelling technique is a powerful aid for judging the future traffic on
existing links and, where possible, should be used in conjunction with axle load surveys.

4.6.3 Project Specific Visual Observation

The manual visual observation technique is simple and inexpensive, and gives acceptable results if a high level of
accuracy is not required. The technique should only be applied to Category C and D roads with design traffic less
than 1 million equivalent standard axles.

The method is based on an assessment of the loading of the heavy vehicles. The recommendations in TRH16
regarding the E80 loading per axle are repeated in Table 21 and are considered adequate. Using these values, the
E80 for predominantly fully laden 7 and 8 axle vehicles is 6.3 and 7.2 E80/HV. These agree well with the values in
Table 18 for long heavy vehicles under normal overloading control. TRH16 provides a template for recording the
data during the visual observation survey.

Table 21. Recommended E80/Axle Loading for Visual Observation Technique
Description of Heavy Vehicle Loading Percent of Vehicles Axle Load Factors
Fully Laden (%) Empty or Partially (E80/axle)
Laden (%)
Predominantly lightly laden vehicles < 35 > 45 0.3
Fully laden, partially laden and empty vehicles 40 – 45 34 – 45 0.5
Fully and partially laden vehicles 60 – 75 < 30 0.7
Predominantly fully laden vehicles > 70 0.9

Section 4: Design Traffic Estimation
Page 48