Page 38 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

4. DESIGN TRAFFIC ESTIMATION
The composition of vehicle traffic on roads is highly varied,
ranging from light passenger vehicles to buses and heavy
vehicles transporting commercial goods. The flow of the Abbreviations used in Traffic Analyses
combined light and heavy vehicles is important for traffic
engineering. However, the pavement design process • AADE: annual average daily E80s
focuses on the response of pavements to loading, • AADT: annual average daily traffic
specifically the volume and loading of heavy vehicles.
• ADE: average daily equivalent E80s
The study and quantification of traffic loading is • ADT: average daily traffic
complicated by its extremely variable nature. There are • ADTT: average daily truck traffic
two modes of land-based transport for commercial goods, • B E : lane distribution factor
rail and road. Shifts in the preferred mode of transport
from rail to road results in changes in the traffic loading. • CTO: Comprehensive Traffic Observations
Economic growth requires good transport infrastructure, Stations
which also affects the long-term growth in traffic volumes. • E80: standard axle load (80 kN per axle)
Sporadic construction activity causes an increase in specific • E80/HV: E80 per heavy vehicle, considering
heavy vehicle types for the duration of the construction all axles
project. Seasonal activities such as harvesting affect the
traffic on certain routes. • ES0.003 to ES100: TRH4 design traffic
classes
Changes to legal axle load limits, and the level of • ET: cumulative equivalent traffic
enforcement also affects the loading and overloading per • EVU: equivalent vehicle units with respect to
vehicle, as do changes in the mechanical design and load lane capacity (e.g., 1 heavy = 3 light)
carrying capacity of vehicles. Therefore, a host of factors • F 7/5 : adjustment of 5 day to 7 day traffic
are considered to properly assess the volume, loading, count
seasonal fluctuation and long-term changes in traffic,
specifically heavy vehicle traffic. • F E : adjustment for exceptional periods
• F S : adjustment for seasonal variation
The wander of heavy vehicles across the width of the lane • HVV: heavy vehicle volume
also affects the loading used in design. A variety of heavy
vehicles on a 3.7 metre wide lane will have a typical wander • LEF: load equivalency factor
pattern that is normally assumed for pavement design. • MESA: million equivalent standard axles
However, a Bus Rapid Transport (BRT) situation has a • n: damage exponent in power damage law
much more concentrated wander, and hence causes a • sdp: structural design period
greater degree of damage to the pavement. Similarly, high
speed heavy vehicles cause short duration loads, while • SHV: short heavy vehicle
slower moving vehicles result in longer duration loads that • WIM: weigh-in-motion
are more severe for many types of pavement layers.

The speed of vehicles also determines the increase in loading caused by dynamic effects. This is normally not
catered for in pavement design, other than specifying minimum riding quality requirements. Bonuses are often paid
to contractors who exceed the riding quality specification because smooth riding surfaces reduce the magnitude of
dynamic loads and, therefore, increase pavement life.

The consequence of an incorrect design traffic estimate is serious and costly. Although the calculations used in the
process of estimating the design traffic are fairly simple, the high uncertainty associated with the inputs often leads
to low confidence in the results.

The loading on port pavements, container terminals, airport
runways and bus lanes are different to the loading on road
pavements, and require careful consideration. It is Airports, Ports, Container Terminals
recommended that design guidelines developed for these
specific pavement applications are consulted when attempting The loading on port pavements, container
the design of these pavements. Examples of appropriate terminals, airport runways and bus lanes are
design guidelines are: different to the loading on road pavements,
and require careful consideration. It is
• Heavy Duty Pavements: The Structural Design of recommended that design guidelines
Heavy Duty Pavements for Ports and Other developed for these specific pavement
Industries. Edition 4. 2007. Interpave, The Precast applications are consulted when attempting the
Concrete Paving and Kerb Association. www.paving.org.uk design of these pavements.

Section 4: Design Traffic Estimation
Page 27