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
4.3 Network Coverage
Reports of AADT and VKT relate to the network where traffic is counted. As traffic counts are
almost always a sample of the road network, the issue of network coverage or link-node
development is important. The key issues are as follows:
Difficulty in prescribing rules like number of permanent sites per km as each RA network is
unique. Good judgement is required to define road sections or links;
Use of performance measures such as an indication of the number of permanent sites for a
given number of homogeneous sections;
Frequency of reviewing uniform traffic sections (UTS) or road sections; and
Definition of homogeneous or uniform road sections.
The accuracy of AADTs at sample stations depends critically on the selection of Pattern Stations,
which alone provide the full year of traffic counts and from which the related short-term sites
depend for their adjustment factors. In order to increase the confidence in the pattern information
gained, the collection of data at all Short-term Stations in a pattern group should be made at the
same time as the related Pattern Station (if it is seasonal) is being sampled.
The accuracy of an AADT estimate depends partly on the reliability of grouping Short-term Stations
with Pattern Stations, and the method of grouping is important. Areas with similar economy,
culture and development tend to show similar traffic patterns, and proximity on its own may not be
a good indicator.
This section deals with those issues and discusses good practices found in each area. The
following sub-sections cover sampling (Section 4.3.1), frequency (Section 4.3.2), defining
homogeneous sections (Section 4.3.3) and directionality (Section 4.3.4).
4.3.1 Sampling
Sampling in a traffic count program relates to two separate yet inter-related issues; the sampling
unit and the frequency of sampling. Both issues are discussed in this section.
Sampling Unit
Pattern stations are best located by randomly selecting the number of sites on roads from each of
a number of AADT strata, within each geographical region. As a general rule, the density of
stations in each stratum should be proportional to the product of the total length of road and the
square root of the mean AADT in that stratum (Austroads 2004).
In practice, the number of Short-term Stations is determined by the extent of the road system and
the availability of funds. The number of Pattern Stations, and the ratio of Pattern Stations to Shortterm
Stations, cannot be determined in advance, because the assessment will depend on the
regional grouping and the reliability of matched patterns. Thus, the achievement of consistent and
specified accuracy levels in AADT estimation is essentially an iterative process that develops over
time and with experience.
The guidelines for the densities of traffic counting stations are shown in Table 10.