A range of reliability values is required to reflect the different major user groups. It is difficult to generalize about the value of reliability as it will be project, location, user, and time-specific. For one project studied, the value of improvements in reliability were found to be negligible, whereas for another project they were found to add 25% to the welfare benefits of time savings achieved. It is important to recognize the importance of disaggregating user values of reliability — the “granularity” of reliability. Different values are placed on reliability by different network users at different times and for different trip purposes. Therefore, a single monetary value for reliability will be of little, if any, use in project appraisal. Practitioners cannot assume that values used in one study are readily transferable to a project in another situation. It is also important to avoid potential double-counting when factoring reliability into project assessment. This can arise if the standard values of time used to assess average time savings already have an implicit, crude value for reliability incorporated in them.
Most of the existing reliability targets can be found in the rail and aviation sectors, transport modes that seek to run to strict timetables. Governments usually oversee supply by monitoring and setting performance standards. Punctuality statistics provide bellwethers for regulatory monitoring and establish a degree of accountability in relation to service quality. Reliability targets and performance indicators for services and infrastructure performance can facilitate discussions between users, operators and decision makers regarding the right levels of reliability. But employing fixed targets may be distorting as they can dominate other service characteristics that may be of equal, or greater, importance. Reliability targets need therefore to be carefully coordinated with other key performance indicators. Such targets also invariably present an average level of reliability not reflecting diversity in the demand for reliability.
There are also trade-offs to be made. For instance, a rail infrastructure manager may enhance reliability by reducing the number of trains that it operates. The improvements in reliability may then come at the cost of a more limited train schedule and higher overcrowding on the trains. Indeed, the incentives that the targets create in relation to other policy goals and the overall efficiency of transport systems need regular review.
Robust and consistent reliability assessments can be developed. Their deployment is important for informing decisions on achieving more optimal levels of reliability on surface transport networks, and for the selection of cost-effective policies and projects. Reliability is unanimously regarded as a desirable transport network attribute and travel time reliability has been found to be an important factor when it has been incorporated into cost-benefit assessment. The importance of reliability is highly case-specific but some recent studies found that incorporating reliability added anything from 10% to doubling the estimated benefits achieved.