Travel time reliability has been put by all stakeholders in the transport arena at the centre of attention: transport planners, operators and users agree on the need to take actions for more reliable transport systems. Although the framework is clear in terms of the stakeholders’ claims, the theoretical knowledge about reliability is even internationally not exhaustive and the same definition of the term is not intuitive and seems to differ from country to country.

As a position-fixing for its activities the working group initiated a small survey within the PIARC community on the international standard of dealing with “reliability in transport systems”. The survey that collects the results related to 18 Countries members of PIARC, provided the following results: a) in terms of definition of the concept, the relation to travel time is preponderant with terms such as, certainty, variability, and punctuality; however, other characteristics of a reliable mode of transport are also citied, such as: user-orientated quality, robustness, availability, efficiency and the quality of real-time information; b) there is still a great deal of disagreement about reliability in transport systems, from definition, through measurement, to calculation and interpretation of results; c) there is a consensus that reliability in cost benefit analysis should be considered to get accurate and correct results, avoiding underestimation of possible benefits; furthermore, there is a wide consensus about the introduction of travel reliability indicators ad KPIs associated to the obligations of the NRAs – National Road Authorities.

In conclusion, travel time reliability is a concept that needs a careful preliminary analysis in order to define the scope of the measurement and the use of the indicators. Since reliability is one of the expected effects in the market of transport services associated to an investment or a specific operational policy, it is necessary to focus on the expected benefits to the users. The improvement of reliability brings about a reduction in travel times, or more precisely a reduction of delays with respect