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

Accessed by AR - ARRB TRANSPORT RESEARCH on 04 Feb 2005
Austroads 2004
— 28 —
Best Practices in Road Use Data Collection, Analysis and Reporting
Table 14 - Suggested volume stratification based on % change in AADT
From AADT or AAWT To AADT or AAWT
(Source: Main Roads WA)
% change above which a new traffic
section is required
40,000 50,000 15%
30,000 40,000 20%
20,000 30,000 25%
10,000 20,000 30%
7500 10,000 35%
5,000 7,500 40%
4,000 5,000 45%
200 400 50%
10 20 100%
Road Authorities deal with a total number of homogeneous road sections varying from 500
segments to 5,000 segments. These segments will change from time to time and various recent
techniques have offered potentially new solutions in the updating of road segments. Thoresen and
Michel (2002) reported on the use of hourly flow histograms from Pattern Stations to develop
default hourly histograms for other sites and ultimately across the entire network. Such a
technique provides, indirectly, another means of deriving homogeneous sections across the
network.
RTA NSW employs an alternative technique in defining a homogeneous section and a network is
divided at points of major flow changes. RTA (2003) proposed a computational solution based on a
statistical set of rules to update UTSs (Uniform Traffic Sections or Segments) automatically with
minimal manual maintenance. The techniques are based on working with an alternative set of
road sections, called ‘Weighted Road Segments’ (WRS) that encompass a number of UTSs and
defining UTSs in terms of changes in AADTs within each WRS by a 95% statistically significant
measure of twice the square root of AADT, based on the assumption that AADT is distributed as a
Poisson variable.
While it is easy to define a road section on AADT volume stratification, the same road section may
fall under a number of road classifications (urban, rural, etc). Furthermore, the definition of the
group changes depending on the vehicle classification. This raises another implication for good
practice - the need to maintain a historical record of changes to homogeneous sections and
classifications over time.
In summary, a good practice among RAs in defining homogeneous sections, and hence the
calculation of AADT and VKT in a statewide counting program, would involve the following iterative
steps:
(a) Locate permanent and short-term count stations based on judgement and road functional
classes (e.g. Table 12);
(b) Collect counts;
(c) Divide road network using volume stratification and traffic pattern behaviour;
(d) Determine the number of stations required by applying a volume stratification technique (e.g.
Tables 13 and 14);