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
(b) Vehicle concatenation – When the space between two consecutive vehicles is small, it
becomes difficult to separate the two vehicles using, say, a standard two axle sensor
classifier. Where very high accuracy is required in a traffic operation (e.g. toll collection), a
loop sensor may be useful in separating vehicles by monitoring the loop occupancy time. The
loop can be placed between the two axles. The software for classification must be capable of
using the information from the loop sensor. An infrared beam can also perform the function of
monitoring occupancy as an alternative to the loop (see (d) below).
(c) Axle sensor array for multi-lane highway – There are situations when short-term classified
counts are needed for each lane on multi-lane highways. An array of axle sensors may be
needed if the equipment is unable to distinguish traffic travelling in each lane, as shown in
Figure 5. By deduction, lane traffic can be determined. For simplicity, only the configuration
for axle or axle-pair counts is shown. An extra parallel set of sensors is needed for classified
counts. An alternative arrangement is to locate an extra data logger in the median. This
counter or logger can monitor one or more lanes of traffic closest to the median. Instructions
from equipment manufacturers should be followed in a study.
tube sensors
and data logger
median
kerb
Figure 5 – A sensor array for separate lane axle counts on a three-lane carriageway
(d) Infrared axle sensor – Infrared beams are routinely used in commerce and industry, e.g. as a
‘doorbell’ in many shops. A beam is aimed from a transmitter to a receiving unit, and any
break in the beam is processed to produce an electric signal, which can be used to
increment a counter or sound a buzzer. In traffic applications a beam may be aimed across a
roadway or reflected from a raised pavement marker. The reflectors can be on the centreline
(counting one direction), or the far side of a two-lane road (counting both directions).
The beams should be close to the road surface as an alternative to a surface-mounted axle
sensor. Potential problems include detector robustness and the maintenance of beam
alignment. The concept does offer the advantage of minimum interference to the pavement
and traffic stream, but it is still to be proven for multi-lane highway applications.