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 Austroads 2004 — 18 — Best Practices in Road Use Data Collection, Analysis and Reporting Figure 7 - Expected percentage error for AADT from ADT and count duration (n-Day counts) for a 75% confidence level. An appreciation of the accuracy of AADT data permits the consideration of the rounding of numbers. Table 7 provides some examples of rounding and compares the percentage change in AADT against the maximum error requirements stipulated in NAASRA (1982). It shows that, in all cases, rounding is within the permitted error range. The maximum error permitted is based on random sampling and rounding should be carried out only if necessary, e.g. for reporting purposes. Table 7 - Examples of rounding AADT AADT Example Rounding Percentage change Maximum error permitted 0-100 46 50 8.7% 50% 101-300 157 200 27.4% 35% 301-1100 678 700 3.2% 25% >1100 2345 2300 1.9% 15% 4.2.4 Adjustment Factors Traffic counts from both permanent and short-term sites often are incomplete. Seasonal adjustment factors, conversion and expansion factors are typically applied to traffic counts in order to statistically convert shorter counts to annual averages. FHWA (2001) describes it as follows: Assuming that temporal characteristics affect all roads and since continuous temporal data exist at several points (the permanent/pattern sites) to describe temporal variation, it is possible to transfer knowledge by developing factoring mechanisms.
Accessed by AR - ARRB TRANSPORT RESEARCH on 04 Feb 2005 Austroads 2004 — 19 — Best Practices in Road Use Data Collection, Analysis and Reporting Most RAs have utilised a range of adjustment factors, as can be seen in Table 8 (Roper et al. 2002). Table 8 - Adjustment factors by jurisdictions Data Item ACT NSW NT QLD SA TAS VIC WA NZ Seasonal Adjustment Factors Expansion factors (e.g. 12 hour count to 24 hour count) Other adjustment factors (e.g. public holiday, weekends, or daily flow factors from control sites) A number of adjustment factors have been applied, in various jurisdictions, which include: Seasonal adjustment factors, Hour-of the day expansion factors, Day-of-week adjustment factor, Lane distribution factors, Growth trends at that location, Factors for short-term counts, Factors for total volume and estimates for trucks, Visual 8, 12 hour expansion factors, Visual truck expansion factors. GTEP Part 3 provides a general guideline, noting that where a road counting program has been established and seasonal patterns identified, the AADT at a particular location may be estimated by multiplying a sample count (say, two to six days duration) by the Seasonal Adjustment Factor derived from a Pattern Station representative of the required location by: (AADT)j = ADT ij × (SAF) i,k where (AADT)j = the AADT at the required location j; (ADT)ij = the sample count in the season (or month, week, day, etc.) i at the location j; (SAF)ik = the Seasonal Adjustment Factor for the season (or month, week, day, etc.) i at a Pattern Station, k, representative of the required location j. As conditions will vary in each jurisdiction, the specific adjustment factors utilised by RAs will each be unique to their jurisdiction. It is good practice for each RA to derive its own adjustment factors. Also, adjustment factors should be clearly detailed, say, in appendices to provide clarity and transparency to the process of adjusting the raw data.
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AP-G84/04 Best practice in road use data collection, analysis ... - WIM

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