Page 53 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

Table 16. Adjustment for Seasonal Variation

Adjustment Factor F S

ADE O
F = (9)
S
ADE I

where ADE 0 = Out of season ADE
ADE I = In season ADE
In Season Traffic Survey
ADE = ADE
I
7
ADE = F ADE
S
7
O

(D ADE + (365 − D )F ADE ) (10)
S
S
7
S
7
AADE =
365
where D S = Number of days with seasonally high traffic in a year
Out of Season Traffic Survey

ADE = ADE
7
O
F S
ADE =
I
ADE 7

D ADE
� S 7 + (365 − D )ADE � (11)
F S S 7
AADE = 7
365
where D S = Number of normal days in a year

4.4.3 Adjustment for Long-Term Variation (Heavy Vehicle and E80/HV Growth Rates)
Adjustment for long-term traffic variation is probably the most likely cause of incorrect design traffic estimates. This
is because it is extremely difficult to anticipate how the traffic volume and loading per heavy vehicle change with
time. In general, a growth in heavy vehicle volume has been observed in South Africa (De Bruin and Jordaan, 2004
and Theyse, 2008c). The general trends show:
• Number of long heavy vehicles increasing.
• Number of short and medium heavy vehicles increasing at a much lower rate, or remaining constant
with time.
• End result is that the total number of heavy vehicles is increasing with time.

There are always exceptions to the general trend. Cases were found where the heavy vehicle volumes remained
fairly constant over a period of a few years. These stations are, however, in areas where the economic activity in the
area has stabilized, such as fully developed agricultural and forestry areas.

The growth rate for heavy vehicle volumes may be derived from past data using Equation (12), if data are available
for a sufficient duration of time, at least 5 years. Typical heavy vehicle volume growth rates were calculated from
historical data collected at selected permanent traffic observation stations in South Africa and the results are
summarised in Table 17 (De Bruin and Jordaan, 2004). The period for which data was available varied from 4 to 9
years. Although the data in the table only cover a very limited set of conditions, it does provide an indication of the
extreme values that may be expected for traffic growth in South Africa.

If the trend in traffic volume growth is difficult to anticipate, the trend in heavy vehicle loading is almost impossible
to anticipate. Generally, because there is a shift toward using long heavy vehicles, the average number of E80/HV
should increase. However, the enforcement of legal axle load limits (or the lack thereof) has a significant effect on
the long-term trend for the E80/HV, combined with the starting E80/HV:
• If the E80/HV starts from a low base on a specific route and there is a lack of overload control, the
E80/HV increases with time.
• If there is a lack of overload control on a route, the E80/HV starts from a high base, but may even reduce
if effective policing is applied.

Section 4: Design Traffic Estimation
Page 42