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
Frequency of Sampling
The issue of frequency of traffic counts relates in particular to the frequency at which short-term
counts are conducted at specific sites. In determining the frequency of data collection, a road
authority will need to consider the rate of change of values of particular data types. The accuracy,
quality and currency of the data should be determined with reference to the cost of collecting those
data and the value and benefit of that data (Western European Road Directorate 2003). The issues
are summarised as follows:
Within a year – number of sampling periods, e.g. once, twice or more times per year;
duration of each sampling period, e.g. 1 h, 1 day, 1 week or more; when to survey, e.g.
randomly, outside school holidays or at each season; consistency with previous surveys;
Between years – predictability and consistency of traffic growth; predictability and
consistency of factors leading to traffic growth; existence or otherwise of commodity in traffic
flows between areas or road types, e.g. rural versus urban or National Highways versus main
roads;
External demands – internal road authority requirement; external government agency
requirements; political or public relations demands.
The frequency of collecting short-term traffic counts varies amongst RAs. The current practices
range from a frequency of a sample once every year to as infrequent as ten years apart. Most RAs
sample at a frequency of once every two to three years. Within this sample program, one half or
one third of the sites are sampled in any year, resulting in all sites covered over a two to three year
cycle (described as a ‘rolling’ program). Increasing budgetary pressures are leading RAs to lower
collection frequencies, e.g. from three to four years to five to six years in South Australia.
Further ‘fine-tuning’ is applied within a sampling frequency regime, by differentiating between
classes of road (urban and rural) as well as within classes of road (mainly rural). Thus, Western
Australia reported a two year metropolitan cycle and a five year rural cycle while Tasmania
reported sampling 100 sites every two years, 300 sites every five years and 450 sites every ten
years.
It is good practice that the sampling interval should not extend beyond three years, under what can
generally be described as ‘normal’ conditions. In practice, sampling frequency is a balance of
costs and accuracy, and recognition of changing traffic conditions, best determined by the users of
the data themselves. As such, for rural roads where traffic counts hardly change, a ten year
sampling frequency may be justifiable, as with some roads in Tasmania. This, of course, would not
apply to rural roads closer to urban areas, where it is likely that conditions will change more rapidly.
The local knowledge of RAs enables fine tuning within portfolios of count stations, to achieve cost
effectiveness in the counting program.
4.3.2 Defining a Homogeneous Section
Homogeneous sections are utilised to transpose AADTs calculated for specific survey locations, as
part of the sample traffic counting program, across to road sections in the total road network. By
assigning an AADT for each road section in the network, a calculation for VKT can be made,
multiplying the road section length by the AADT and adding up across all road sections.
Homogeneous sections are groups of roads or ‘strata’ or ‘uniform traffic segments’ (UTS), each of
which includes a relatively large number of segments that can be regarded as a ‘population’ for
statistical sampling. Homogeneous sections also relate to groups of roads to which adjustment
factors, described earlier, can be applied. FHWA (2001) describes the creation of ‘factor groups’
(similar to homogeneous sections) as follows: