Quantifying the full reliability benefits of road network improvements

Assessing travel time reliability and the robustness of networks (especially road networks in major urban areas) is becoming more important as networks become more vulnerable. Especially in the Netherlands, the interconnectivity of networks of different scale is low and the level of usage is high, which leads to low spare capacities for unfavorable conditions. Also, the number of incidents is high and increasing. The Randstad area, lying between Amsterdam, The Hague, Utrecht and Rotterdam, experiences about 1750 incidents a year with a clearance time of over one hour. Already small disturbances can cause major disruptions on large parts of the network. As such we can expect that major benefits could be gained from measures that improve the stability of operating conditions of the road network under unfavorable circumstances. In 2008 the Dutch Ministry of Transport, Public Works and Water Management published the “MobiliteitsAanpak” (Mobility Approach) (Ministry of Transport, Public Works and Water Management, 2008). This policy document proposes investments in the Dutch road, rail, regional public transport and waterways network to improve travel times and their reliability and reduce negative external effects of transport. By assignment of the Dutch Ministry of Transport TNO evaluated the benefits of the Euro 30bn’s worth of investment in the road network between 2020 and 2028. New in this analysis was that, in addition to travel time gains, we assessed the full reliability benefits of transport projects, where, next to the small travel time variations we also included the effects of major disruptions, where the robustness of the transport network is critical. Traditionally benefits related to improved travel time reliability were assessed using crude rules of thumb which are not related to the present or the future state of the network. In the Netherlands, a 25% markup is applied to travel time benefits. This number is based on Quantifying the full reliability benefits of road network improvements Snelder, Maaike; Tavasszy, Lori 12 th WCTR, July 11-15, 2010 – Lisbon, Portugal 2 expert judgment. Instead of using this fixed markup we chose to assess the benefits of improved robustness of the network by using Monte Carlo simulation to compute the reduction of travel time losses under disturbed travel times (reliability analysis). We included both small, frequent and large, infrequent variations in travel time. Furthermore, we proposed a second method that focuses more on the effects of large disturbances (vulnerability analysis). One important outcome of the analyses is that the reliability benefits of the proposed road projects are similar in magnitude as the benefits from reduction of average travel times. We also find that the outcomes are overall positive for the complete set of measures, but that effects can be negative in individual cases. In other words, there are specific situations in which the travel times become less reliable for certain travelers, despite a clear gain in average travel time under normal operating conditions. As the projects were not evaluated before with robustness in mind, an important implication of this is that the proposed set of projects in the Dutch road network strategy can be optimized further, bearing in mind that a trade-off between travel time and reliability may lead to different projects. Finally, we showed that splitting through traffic from local traffic, upgrading the secondary road network and completing ring roads makes the network more robust against large disturbances. Of course, the extent to which the robustness improves depends on the amount of spare capacity that is created, the usage of the roads under regular conditions and the distance between the original route and the fall back options.