Page 61 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

4.6.4 Project Specific Traffic Surveys

The main purpose of a traffic survey is to obtain estimates of the base year HV volume and E80/HV. A number of
survey options are possible, each requiring different levels of effort and cost, and yielding different levels of detail.

4.6.4.1 Manual or Electronic Counts with Limited Static Weighing
This technique is applicable to Category C and B roads, when the traffic volumes are relatively low, i.e., less than 300
heavy vehicles per day. The traffic volume from electronic 7 day surveys is combined with limited static weighing of
selected vehicles to determine the E80/HV. It must be determined whether the survey was done during seasonally
high traffic or exceptionally low traffic. The static weighing is done using portable scales which should be placed in a
safe, but accessible location. The static weighing process is limited to daylight hours for safety reasons, but should
be done for at least a 12 hour period from 06h00 to 18h00. The team doing the survey must ensure the random
selection of the heavy vehicles for static weighing, as there is normally a tendency to target the heavily laden
vehicles. The methods of weighing are discussed in Section 4.2.

4.6.4.2 Dynamic Weighing or Weigh-In-Motion
This method is recommended for Categories A and B roads carrying high volumes of traffic. The survey should be
conducted for at least 7 days, and it must be assessed whether the survey was done during seasonally high traffic or
exceptionally low traffic.

The axle load of individual axles is recorded under normal operating conditions. This type of survey is generally
referred to as a weigh-in-motion (WIM) survey (see Section 4.2). The axle load measurements are done at the
operating speed of the heavy vehicles, which impacts the results. Vehicles moving at speed tend to bounce and roll,
while the wheels hop at a high frequency. This results in an axle with a fixed static weight causing a spread in
results when weighed dynamically. For this reason, the approach to the WIM installation should be smooth and the
pavement structure sound. The WIM must also be properly calibrated to get a reliable estimate of the static axle
load. Referred to TMH3 (1988) for detailed survey requirements.

Two types of error are associated with the results from dynamic axle load surveys:
• Systematic error or bias: deviation of the observed dynamic axle load from the actual static load.
• Random error: spread of the observed dynamic axle load for a fixed actual static load.

Statistical techniques have been developed to correct the errors (Prozzi et al, 2006 and Theyse et al, 2007), but can
only be applied if the level of error is known. The correction for random error has been shown to have a minimal
effect on design traffic estimates. However, a 5% WIM bias may result in a 20% error in the design traffic estimate.
The proper calibration of WIM equipment is, therefore, of the utmost importance.

4.7 Design Traffic Calculation
The steps involved in a design traffic calculation are shown in a flowchart in Figure 27.

(i) Worked Example of Traffic Calculation
A worked example of calculating traffic from published data is shown in Table 22. The data are obtained from the
CTO Station in Figure 26.

WIM Calibration Know Your Traffic Data!
A small bias in WIM measurements When calculating design traffic from
can result in a large error in the available or measured traffic data, it is
design traffic estimate. The proper essential to know what the data is for.
calibration of WIM equipment is For example, if it’s for the entire road,
therefore of the utmost importance. directional and lane distributions need to
be applied.

Section 4: Design Traffic Estimation
Page 50