Page 52 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

Table 15. Adjustment for Exceptional Periods

Adjustment Factor F E

ADE E
F = (6)
E
ADE N

where ADE E = ADE for exceptional days
ADE N = ADE for normal days

Traffic Survey During Normal Period
ADE = ADE
7
N
ADE = F ADE
E
7
E

(D ADE + (365 − D )F ADE ) (7)
N
7
E
N
7
AADE =
365

where D N = Number of days with seasonally high traffic in a year
Traffic Survey During Exceptional Period

ADE = ADE
7
E
ADE 7
ADE =
N
F E

D ADE
� N 7 + (365 − D )ADE �
F N 7 (8)
AADE = E
365

where D N = Number of normal days in a year

4.4.2.4 Adjustment for Seasonal Variation
The adjustment for seasonal variation is done similarly to the adjustment for exceptional periods, but with the
seasonal adjustment factor F S . The calculation of F S is given in Equation (9) in Table 16, along with the calculation of
the adjusted AADE (Equations (10) and (11)). F S is normally equal to or less than one.

The adjustments for exceptional periods and seasonal variation are done consecutively. The AADE output from the
adjustment for exceptional periods is the ADE input of the sample period (ADE 7 ) for the seasonal variation correction.
(i) Study of the Effect of Harvesting Season
In the study investigating the effect of short-term traffic variations (Theyse, 2008c), extensive data were not
available to calculate the value of the seasonal adjustment factor F S and the duration of seasonally high traffic. A
yearly set of WIM data was, however, available for one location where sugarcane is harvested. The E80/HV was 2.5
for the seasonal harvesting period, compared to 2.0 for the rest of the year. The traffic volume increased from 419
to 449 HV/lane/day during the harvesting season.

If a 7 day traffic survey is done during the non-harvesting season and the ADE is not corrected for the higher traffic
volume and vehicle loading during the harvesting season, the result is a 9% underestimation of the design traffic
over a 20 year period (assuming 6% traffic growth rate). Such an error may result in the road lasting for only 18
years and not the intended 20 years. Again, the difference is not dramatic, but it may be more severe for other
cases. It is recommended that the potential for seasonal traffic loading be carefully investigated. If it is likely to
occur, the traffic survey should be conducted in and out of season. Alternatively, if the adjustment for seasonal
traffic is neglected, the survey should be done during the high season, to ensure a conservative error.

Section 4: Design Traffic Estimation
Page 41