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
With the prevalent use of single axle sensor for traffic counts, it is important that the number of axle
pairs obtained be corrected for vehicles with more than two axles. The correction factor is
dependent on traffic composition and site conditions and should be calibrated on-site. Some
guidelines on the correction factor are given in GTEP Part 3. For example, if heavy vehicles
account for 12% of the total traffic on an urban highway, the number of axle-pairs obtained should
be reduced by about 6%, i.e. the correction factor is 0.94 ±0.15 at the 95 % confidence limits.
There are no standard practices amongst RAs regarding when axle pair counts are collected rather
than classified counts. At the Austroads Road Use Data Workshop, there was general agreement
that classified counts be collected wherever possible. As mentioned earlier, a pair of axle sensors
is essential for classified counts in spite of higher installation and maintenance costs. An axle
sensor pair is also more appropriate than a single axle sensor in obtaining bi-directional counts as
traffic volume increases.
Table 1 recommends what could be used under different axle arrangements and AADT values.
In Table 1, the threshold AADT values correspond to different Levels of Service (LoS) for two-lane
rural roads and arterial roads (GTEP Part 2, Austroads 1988b; TRB 2000). At a LoS of E, a road
facility is operating at capacity. Usually a facility is designed for a LoS of C. Delay is frequently
experienced at LoS of D and E. The values in Table 1 are for a peak-hour to daily factor of 0.15.
Road environment
Combined direction (e.g. two-lane
two-way rural roads)
Separate direction (e.g. multi-lane
carriageway at 80 km/h speed limit)
Table 1 - Use of axle sensors under different AADT values
Indicative maximum AADT (veh)
One axle Two axles
9,000 (two lanes)
(Level of Service = D)
8,500 per lane
(Level of Service = C)
15,000 (two lanes)
(Level of Service = E
or at capacity)
11,000 per lane
(Level of Service = D)
On multi-lane highways, temporary surface mounted pneumatic tube sensors may not be suitable
under heavy traffic conditions, poor pavement surface or OH&S problems for field staff. The count
accuracy also decreases with increase in traffic flow and the number of lanes that an axle sensor
(or an axle pair) has to cover. In these situations, it could be better to use in-ground loop sensors
described in Section 3.2.
The development of piezo-cables for axle detection continues to progress (Luk and Brown 1987;
ARRB TR 2003). The cost is higher than pneumatic tubes but should be able to offer a working life
similar to that of an in-ground inductive loop - four to five years depending on traffic volume, its
composition and pavement type and condition. Piezo-cables can be installed lane-by-lane so lanebased
counts are possible. The piezo-sensor needs to cover only half to three-quarters of a lane
width to reduce cost and possibly improve accuracy with ‘cleaner’ axle actuations.
Axle sensors give more distinct axle actuation times and therefore better accuracy than loops in
speed measurement and vehicle classification. Figure 2 shows the four timing pulses for speed
calculation and hence vehicle classification. For very slow or stationary traffic, it is difficult to
separate vehicles using the axle actuation times T1 to T4 and determine speeds and classify
vehicles, especially if a vehicle straddles over both axle sensors for a long period of time. Inductive
loops and the related electronic circuitry are designed to monitor loop occupancy (i.e. the time that
a vehicle is on top of a loop) and are more suitable for slow traffic, e.g. urban traffic in peak hours.