Page 47 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

Figure 21. Typical WIM setup

4.2.2 Vehicle Classification
The appropriate vehicle classification system depends on the traffic data required and the capabilities of the traffic
monitoring equipment. Heavy axle loads, associated with heavy vehicles, do most of the damage on pavements.
For pavement design, traffic should therefore be split between light and heavy vehicles.

A number of heavy vehicle classification systems are available. The common systems used in South Africa are
summarised in Table 12. For classification systems using the number of axles per vehicle, traffic measurements are
made using an axle load sensor in combination with loop sensors (see Section 4.2)

The extended heavy vehicle classification system is based on the length of the vehicle rather than the number of
axles. The definition of the heavy vehicle classes is:
• Short heavy vehicle: length < 10.8 metres
• Medium heavy vehicle: length between 10.8 and 16.8 metres
• Long heavy vehicle: length > 16.8 metres

The Extended Light/Heavy vehicle classification system is commonly used for estimating design traffic.

4.3 Daily Equivalent Traffic Parameters
4.3.1 Average Daily Traffic (ADT)

Traffic is most often denoted by the Average
Daily Traffic (ADT). The ADT represents the
number of vehicles on a road in both Daily Equivalent Traffic Parameters
directions and on all lanes, and includes heavy • ADT is the average daily traffic, and includes all vehicles
and light vehicles. (light and heavy) travelling in all directions.
• AADT is the average annual daily traffic and includes all
4.3.2 Average Annual Daily Traffic vehicles and all directions. The total year’s traffic is divided
(AADT) by 365 days. Heavy vehicles are often given as a
The AADT is calculated by extrapolating the percentage of the AADT or ADT.
ADT to the full year, and dividing by 365 days. • ADE is the average E80s per lane per day over the duration
The heavy vehicles are often given as a of the survey period, calculated using Equation (4).
percentage of the total ADT or AADT. • AADE is the total E80s per lane for one year, divided by
365 days. ADE is extrapolated over the year to get AADE.
Care must be taken to ensure the ADE survey period is
representative of the full year’s traffic.

Section 4: Design Traffic Estimation
Page 36