Monitoring and modelling travel time reliability

The use of reliability of trip arrival times does not normally form part of network performance monitoring and modelling. Reasons to monitor and model reliability of trip times include:
(a) monitoring the performance of road network;
(b) monitoring the performance of public transport networks and services; and
(c) evaluating future options.

Although some national guidelines point the way regarding reliability valuations (UK, New Zealand and Australia), current practice is somewhat lagging behind recommended approaches. There is considerable evidence from stated preference survey results related to demand estimation for toll roads and public transport projects, that traveller’s willingness to pay, extends to reliability of travel time, especially for time-sensitive trips.

Modelling reliability
There are two main approaches to network reliability analysis, namely: using the engineering concept of probability of non-failure; and the concept of variability of travel time leading to unexpected delays to users.

The Florida Department of Transportation method, defines reliability of a network as the percent of travel that takes no longer than the expected travel time plus a certain acceptable additional time. The California buffer time reliability method uses minutes of extra travel time needed to allow the user to arrive on time. The buffer time is calculated as the difference between the average travel time and the upper limit of the 95% confidence interval of average travel time.

A suggested approach to modelling trip time reliability using strategic model outputs is:

Step 1: using the volume to capacity (v/c) ratio, assign a reliability ‘rating’ to each link and each node in the network.

Step 2: estimate the overall reliability rating for each origin-destination pair by summing the reliability for all individual links and nodes used in the assignment of those trips. When more than one path is used for an origin-destination movement, the weighted reliabilities would need to be estimated. As an approximation, the least cost path only could be used.

Valuing reliability
The willingness to pay for reductions in the day-to-day variability of travel time is referred to as value of reliability (VOR). Some US studies have found that users place a value on travel time variability of more than twice the value placed on the average travel time.

There is a significant body of public transport related research which confirms that unexpected delays are perceived differently from recurring delays due to peak period congestion. The evidence suggests that unexpected in-vehicle delays should be valued at 2 to 3 times in-vehicle time. Unexpected delays whilst waiting at public transport stops should be valued at around 5 times waiting time.

In addition to having a value to users, in terms of travel time certainty and travel time reductions due to reduced in average trip times, reliability has an indirect impact on trip costs, by potentially reducing fuel consumption, vehicle emissions and public transport operating costs.

Useful References
Al-Deek, H. and Emam, E. (2006). New methodology for estimating reliability in transportation networks with degraded link capacities. Journal of Intelligent transportation Systems, 10 (3), 117-129.
Ferreira, L. (2007).
Monitoring, modelling and valuing travel time reliability. Portfolio Transport Modelling Team, Queensland Transport, October 2007.
FDOT (Florida Department of Transporation) (2000).
The Florida reliability method: In Florida’s mobility performance measures program.
Lomax, T., Schrank, D., Turner, S. and Margoitta, R. (2003).
Selecting travel time reliability measures. Texas Transportation Institute and Cambridge Systematics.
Land Transport NZ (2006).
Economic evaluation manual, Volume 1, October 2006. Land Transport New Zealand.

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