Page 57 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

4.6 Traffic Investigations for Pavement Design

Traffic data are required for network level planning and management, and project level pavement design. This
guideline focuses on project level analyses. An incorrect design traffic estimate results in the same design risk as a
pavement structure of inadequate structural capacity. The allocation of funds to pavement and material
investigations often enjoy preference to spending funds on traffic surveys. Funds spent on a project specific traffic
survey are, however, well worth the investment for reducing the design risk. One-day traffic counts should only be
used for design with caution.

Four traffic parameters have a dominant effect on the accuracy of the design traffic estimate:
• Daily heavy vehicle volume per lane for the base year
• Heavy vehicle volume growth rate
• E80/HV for the base year
• E80/HV growth rate

TRH16 provides for a number of methods of estimating or measuring these parameters:
• Published results or results from other projects related to the design project in terms of:
− Similar loading of heavy vehicles
− Linked to the heavy vehicle classification
system
− Based on a road stratification system Road Stratification System
• Transportation planning models A road stratification system is used to classify roads into
different strata, with each stratum essentially having the
• Estimations based on project specific visual same traffic characteristics. Sampling can then be done
observations by measuring the traffic characteristics of a few roads in
• Project specific traffic surveys each strata, and applying the measured characteristics to
all the roads in that strata.
The level of effort and cost associated with these
methods increases from the use of known results to the project specific surveys, but so does the value of the results
obtained. The application of these techniques is linked to the importance of the road. Table 20 recommends the
minimum level of investigation based on the road category.

Table 20. Recommended Traffic Investigation Levels for Road Categories

Traffic Parameter
Road Base Year HV HV Volume Growth Base Year E80/HV E80/HV Growth Rate
Category Volume Rate
1
A Traffic surveys Transportation models Traffic surveys Published results
Transportation models
B Traffic surveys Traffic surveys Published results
Published results
2
C Visual observation Published results Visual observation Published results
3
D Published results Published results Published results Published results
Notes
1. Project specific traffic surveys
2. Project specific visual observation and tabulated values
3. Published results, or results from other projects with similar traffic characteristics

For a specific project, the necessary data to derive the base year HV volume, E80/HV, AADE growth rate and E80/HV
growth rate are seldom available. If historical axle load data and traffic volume data are available for a route, these
are used to derive the necessary input. Lacking such historical data, a project specific traffic survey should be done
for Category A and B roads. However, given the once-off nature of the survey it is not possible to derive historical
trends and estimate future growth rates. A stratification system, or results from other projects with similar traffic
characteristics, needs to be used to derive growth parameters.
Lacking this type of information, it is recommended that the
historical growth rates listed in Table 17 are used as a guide.
Justification for Traffic Surveys
The base year HV volume and E80/HV are easily derived from Given the relatively low cost of traffic surveys,
project specific traffic surveys. Given the relatively low cost of compared to the construction and
traffic surveys compared to the construction and rehabilitation rehabilitation cost, it is highly recommended
cost, it is highly recommended that project specific traffic that project specific traffic surveys are done as
surveys are done as part of rehabilitation investigations. part of rehabilitation investigations.

Section 4: Design Traffic Estimation
Page 46