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Chapter 4
Methodological issues in
ex-post evaluation
A number of methodological issues in relation to this case study are
discussed below.
Treatment of intersections
The WARES system was developed for analysing open road conditions and has
no allowance for the effect of intersections. There were quite a few intersections
in the Northam townsite affecting the flow of GEH traffic through the town. In
order to capture the delay effects caused by these intersections, the MRWA (1999)
evaluation increased the length of the base case townsite route by 69 per cent, from
13.6km to 23.0km. 3 While this approach was intended to address the issue of the base
case traffic delay caused by intersections, it distorted the calculation of VOCs and
crash costs.
The MRWA’s analysis of effects of intersections on traffic delay could have been done
in other ways. For example, data and resources permitting, the delay effects could
have been formally modelled by using a computer program such as SIDRA 4 . In case
of data and resource constraints, explicit assumptions could have been made about
the number of stops and the average delay of each stop for the base case. Sensitivity
testing could have been undertaken to show the effects on results of a range of
delay assumptions.
For the ex-post evaluation in this case study, no formal intersection modelling was
undertaken, nor was there any a need to do so. This was because SHAWMAC (2003a)
provided information on ‘before’ and ‘after’ actual trip times for both the bypass route
and through-town route, and the derived actual travel speeds for the base case route
could be used as a basis to formulate a change in the speed limits. Reducing speed
limits in WARES suppresses the maximum speeds along the main streets, which
serves as a proxy for impedance caused by intersections.
The presence of intersections in the through-town route would cause stop/start
conditions for traffic, which would add to VOCs in the base case. Such effects could
have been modelled separately if the required information had been available.
Because there was no readily available information on the average VOCs per stop,
this ex-post evaluation was unable to estimate the additional VOCs caused by
intersections for the base case. This would lead to an underestimation of VOC savings
in the ex-post evaluation.
3 According to the WARES User Manual (MRWA 1995), the cost of intersection delays can be approximated by adding
150 metres to every link approaching signals and 250 metres to every link approaching a stop or giveway sign.
4 Signalised & Unsignalised Intersection Design and Research Aid.
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