RESEARCH APPROACH ABOUT RELIABILITY OF ... - BHLS - HOME

Three main measures are used for valuing reliability:
- value of delay minutes (average minutes’ lateness);
- reliability ratio (variance approach);
- scheduling costs.
The main approaches to estimate the value of reliability are functions of delay. The
following are some of the available models:
The mean delay approach, incorporates either delays or expected delays into the
estimated utility function. The approach focuses on delays relative to schedule and
therefore is only applicable to public transport. The model equation is::
Utility = T + λ E(DM)
where T = scheduled travel time; E(DM) = expected delay minutes after schedule;
λ= model parameter.
The variance delay approach attempts to value variability in travel times explicitly
by incorporating it into an estimated utility function. The main measures of
variability used are standard deviations and coefficients of variation. The variance
delay approach is commonly applied, perhaps because it is relatively easy to
implement and it produces reliability ratios.
The reliability ratio is commonly associated with studies where respondents are
presented with representative trips in a stated preference format. To calculate the
reliability ratio, researchers estimate a utility function and then divide the
coefficient on the standard deviation of travel time (generally) by the coefficient of
travel time. The reliability ratio can be easily used to value improvements in
transport reliability. The model equation is:
Utility = T + λ f(S)
where T = scheduled travel time; f(S) = Standard Deviation (SD) or coefficient of
variation of travel time; λ = model parameter.
The scheduling cost approach directs attention away from actual variability and
towards the costs of variability, i.e. the costs associated with being early or late.
The scheduling cost approach presents respondents with a Preferred Arrival Time
(PAT) (e.g. a time when they want to be at their destination) and gives them a
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