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

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Accessed by AR - ARRB TRANSPORT RESEARCH on 04 Feb 2005 Rural Type of road Table 10 - Densities for traffic counting stations Austroads 2004 — 24 — Best Practices in Road Use Data Collection, Analysis and Reporting Counting station density (km/station) Pattern Stations Permanent* Seasonal* Freeways, arterials 100-200 200-300 Local roads 5,000-9,000 1,000-2,000 Urban Freeways, arterials 20-50 30-50 Collectors, distributors 100-150 50-100 Local roads >1,000 4,000-6,000 * lower figures (or higher densities) applicable if relatively few Seasonal Stations used Short-term Stations Up to 25 (usually 13-17) Short-term Stations may be characterised by one Pattern Station In a recent review of traffic counting stations, Roper (2001) found that RAs generally exceed the guidelines in Table 11 for traffic counting station densities. In most cases, densities were higher than the guide of 100 – 200 km for rural road and 20 - 50 km for urban count stations. Rural Urban Table 11 – National highway traffic counting station density by location and status Counting station type Station density (km/site) (location and status) NSW VIC QLD WA SA TAS NT ACT NZ Permanent 68.3 68.1 19.3 217.6 221.9 51.0 175.4 6.3 111 Temporary 10.3 6.4 74.1 25.8 35.0 2.7 202.4 9.5 9.8 Overall 9.0 5.9 15.3 23.1 30.3 2.6 94.0 3.8 9.1 Permanent 2.9 2.6 3.2 -* 43.5 7.0 3.3 - 22.5 Temporary 3.0 4.2 20.6 2.8 7.3 0.5 3.9 - 4.5 Overall 1.5 1.6 2.8 2.8 6.2 0.5 1.8 - 3.1 * there are 40 sites in metropolitan Perth although none on National Highways due to the urban form of Perth Because judgemental sampling is often employed to locate Permanent and Short-term Stations, the AADT values obtained cannot be regarded as random samples. The estimation of VKT from these AADT values is therefore not as representative as those obtained by, say, the sampling frame of the Survey of Motor Vehicle Usage (SMVU) of the Australian Bureau of Statistics (ABS 1963 and other years). The VKT from a road authority counting program (e.g. the NSW program) is comparable to SMVU estimates if VKT from all road types in a State are accounted for, especially the VKT from local roads. The NAASRA (1982) method of calculating the number of counting stations to determine the VKT of a region is shown in Appendix B. The method has been developed further in Botterill and Luk (1998) by including accuracy and statistical significance requirements and relaxing the requirement on preset AADT strata. Using the original method, the number of stations required could be underestimated. The derivation of the new formulae is quite involved and will not be included in Appendix B. In any case, as mentioned previously, the provision of counting stations is more than generally recommended in NAASRA (1982).
Accessed by AR - ARRB TRANSPORT RESEARCH on 04 Feb 2005 Austroads 2004 — 25 — 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:
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