A practical method to estimate the benefits of improved network reliability : an application to departing air passengers

This paper develops a practical approach to estimate the benefits of improved reliability of road networks. We present a general methodology to estimate the (changes in) scheduling costs due to (changes in) travel time variability for car travel. We focus on situations where only mean delays are known, which are the typical output of a standard transport model. We show how to generate travel time distributions from these mean delays, which we use to estimate the scheduling costs of the travellers, taking into account their optimal departure time choice. We illustrate the methodology for car access by air passengers to Amsterdam airport and show how improvements of the highway network lead to shorter expected travel times, lower travel time variability, later departure times and reduced access costs. We find that on average the resulting absolute decrease in access costs per trip is small, mainly because most air passengers drive to the airport outside the peak hours. However, the relative reduction in access costs due to improvements in network reliability is substantial. For every 1-Euro reduction in mean travel time costs, there is an additional cost reduction of about 0.75-0.85 Euro due to lower travel time variability and hence lower scheduling costs. Our results thus show that the total benefits from infrastructure improvements are about 80% higher when benefits due to better reliability are taken into account in addition to the savings in mean travel time alone.