AP-G84/04 Best practice in road use data collection, analysis ... - WIM

Accessed by AR - ARRB TRANSPORT RESEARCH on 04 Feb 2005
Austroads 2004
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Best Practices in Road Use Data Collection, Analysis and Reporting
Figure 3 - Typical response of a loop detector crossed by a passenger car
Two types of loop designs are commonly used by road authorities for vehicle counting on a lane of
traffic flow. A loop for counting at mid-block is often a square loop of 2 m × 2 m, and a pair of
these loops at a known distance (4 - 5 m) apart is usually used for speed measurement and
vehicle classification. A second loop design is the SCATS loop for signal operation and traffic
counting. The SCATS loop has a width of 2 m and an overall length of about 4.5 m and is located
near the stopline of an approach. It provides reasonably accurate counts over a measurement
period of, say, 15 minutes but the counting accuracy is less over the length of a signal cycle of one
to two minutes due to the loop location and length.
For accurate counting, care needs to be taken in selecting the size, shape and positioning of the
loop within the traffic lanes. Over-counting can occur when loops in adjacent lanes are too close
and the same vehicle is detected in both lanes. On the other hand, motor cycles or small vehicles,
straddling both lanes may be missed by loops placed too far apart.
The ability of the loop detection circuit to hold the change in inductance (or electric charges) allows
a loop sensor to be a presence detector. This property allows a loop sensor to measure volume
and speed at very low speed and is therefore suitable for congested urban traffic conditions.
However, speed measurement using loops is not as accurate as axle sensors due to more
uncertainty in the actuation timings. This also affects the accuracy of vehicle classification using a
pair of loops. The classification has to be by vehicle lengths and in a smaller number of classes or
bins (see Section 3.3). Also, inductive loop detectors are not suitable for unsealed roads in
general.
3.3 Vehicle Classification System
Vehicle classification data is important for all transport engineering applications. Comprehensive
classification data is now possible using new technologies. Vehicle classification surveys are
concerned with the distribution of traffic data into different types of vehicles. Classified data can be
used for design and performance of road pavements and bridges, traffic capacity and operations
analysis, vehicle categories for traffic legislation and regulation, road safety studies, and economic
studies.