Page 51 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

4.4.2.1 Variations between Day and Night Traffic
If a twelve hour traffic survey was done, typically between 06:00 and 18:00, the counts are adjusted for night-time
traffic. Typically about 70% of the daily traffic loading uses the road during the 12 hour period. However, this
proportion varies between 40% for rural routes and 90% for urban freeways (TRH16, 1991). Projections of the 12
hour survey to 24 hour data should be done using local knowledge, and if possible, by using classification counts
over a 24 hour period. Classification counts usually provide the number of vehicles of each type for each hour of the
day and night. Assuming that the vehicles are similarly loaded during the day and night, the survey data can be
extended to give the ADE.
4.4.2.2 Variations between Weekday and Weekend Traffic

Traffic loading is normally lower during weekends than on weekdays. If the ADE is determined from a survey done
during the 5 day work week only, then the ADE needs to be adjusted to a full 7 day week. This is done using the
factor F 7/5 , given in Equation (5).

ADE 7
F 7/5 = (5)
ADE 5

where ADE 7 = ADE for a full 7 day week
ADE 5 = ADE for a 5 day work week

Unfortunately, in a design situation, the ADE 7 , and therefore F 7/5 , is unknown. Therefore, published values of F 7/5 are
generally used. TRH16 recommends a value between 0.7 and 1, where:
• F 7/5 = 0.7: no additional heavy vehicle traffic over a weekend.
• F 7/5 = 1.0: traffic volume during the weekend and work week is the same.

Since TRH16 was published, some additional F 7/5 values have been recommended (Theyse, 2008c), and are given in
Table 14.

Table 14. Recommended 5 to 7 day ADE Conversion Factors (F 7/5 )

Route Description Weekend Traffic Volume (%) F 7/5 Factor
Range Recommended
Value
Routes carrying mainly agricultural traffic in rural areas 14 to 16 15 0.85
and urban routes with high traffic during weekdays
Routes carrying industrial and mining traffic 13 to 19 17 0.86
Routes connecting major centres and export routes 18 to 21 19 0.89

4.4.2.3 Adjustment for Exceptional Periods
The adjustment for exceptional periods is done with the exceptional period factor, F E. F E is less than one. The
calculation of F E , and AADE using F E , are shown in Equations (6), (7) and (8) in Table 15.

Based on weigh-in-motion data collected at various stations in South Africa, the only period of exceptionally low
heavy vehicle traffic activity is from middle to end December. Depending on which day the Christmas and New Year
holidays fall, the duration of this exceptional period varies between one and two weeks. Investigations showed that
the error in ignoring this exceptional period is small for both AADE and when projected over a 20 year period.
(Theyse, 2008c)

Adjustments for Exceptional Periods
In South Africa, the only period with
exceptionally low traffic is over the Christmas
and New Year period. Investigations showed
that the error in ignoring this exceptional period
is small for both AADE and when projected over
a 20 year period.

Section 4: Design Traffic Estimation
Page 40