Page 46 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

4.2 Methods of Traffic Measurement and Classification

4.2.1 Traffic Measurement
Traffic counts and classification are done manually or are automated using traffic counting stations installed on the
road. Traffic counting is performed on a special or project basis (short term), or on a temporary (medium term) to
permanent (long term) basis by road authorities, as part of their road management system data collection strategy.
Whilst permanent stations provide a continuous traffic record from one year to the next, temporary stations are used
on a sampling or periodic basis to collect data over a minimum specified time period.

Depending on the application, different equipment and combinations of equipment or systems are used to provide
the required data in the most cost-effective way. Installation, implementation and maintenance of these systems are
normally done by specialist service providers. The information provided here is therefore introductory.

Traffic counting and classification is performed at various levels of detail. Primary sensors are the essential sensors
to detect vehicle presence and are also used for basic vehicle classification. By adding secondary sensors, more
information is recorded and a more detailed classification undertaken.

Pneumatic tubes, induction loops, and piezo-electric sensors are typical primary sensors. Induction loops are the
most common primary sensor used and detect the presence of a vehicle by electromagnetic induction. Inductive
profiles are used to distinguish light from heavy vehicles using a single loop. A dual loop system provides inductive
profiles plus vehicle length, and can therefore be used to classify vehicles as short, medium and long.

Piezoelectric sensors, or axle sensors, can be used as traffic counters and vehicle classifiers. They are normally used
as secondary sensors together with induction loops. Classification is based on axle spacing profiles, therefore the
number of axles per vehicle.

Weight measurement and classification is done through weigh-in-motion (WIM) systems. These devices measure the
dynamic axle mass of a moving vehicle to estimate the corresponding static mass. WIM systems can be divided into
high speed weigh-in-motion (HSWIM) and low speed weigh-in-motion (LSWIM). The function of these systems
differs in that HSWIM attempts to quantify actual loads applied by moving vehicles and is an effective way to
measure the entire spectrum, but at lower accuracy. LSWIM can be used to measure samples or pre-screened
vehicles at a higher accuracy, and is, therefore, used for law enforcement.

Different WIM technologies exist. The most common types of mass sensors are piezoelectric cables, load cells, and
bending plates. A complete WIM installation typically consists of at least one mass sensor and two induction loops.
A typical WIM setup is shown in Figure 21. The mass sensors are briefly described below, in order of increasing life
expectancy, accuracy and cost.
• Piezoelectric sensors: These strips essentially consist of copper wire surrounded by piezoelectric material,
which produce a charge when subjected to pressure. The waveform current is proportional to the axle mass.
• Bending plates: A steel plate is instrumented with strain gauges at critical positions to measure strain induced
by a vehicle. The strain is analysed to determine axle load.
• Load cells: These sensors typically consist of one or more steel platforms, beams or plates that are simply
supported by a load cell on each corner to measure the applied force.

Site selection and location characteristics are fundamental to the performance of WIM systems and the selection of
an appropriate site is based on experience and intuition. Apart from pavement characteristics (installation site and
approach), vehicle and climatic characteristics impact on the reliability and accuracy of measurements. Furthermore,
to determine the equivalent static axle mass of a moving vehicle, the system output needs to be properly calibrated.
Whilst calibration requirements vary among different WIM systems, routine calibration is needed.

Section 4: Design Traffic Estimation
Page 35