To limit the costs for the new 2nd Coentunnel, steeper grades are considered than the maximum values mentioned in the guidelines: 6% instead of 4.5%. Steeper grades reduce the length of the tunnel and its entrance road sections, and save construction costs. On the other hand, steeper grades may result in a decrease of traffic safety and traffic flow rate. This contribution focuses on determining the optimum vertical alignment in tunnels, taking into account traffic safety, traffic flow, construction costs and operation costs. The results of traffic flow and traffic safety calculations are used in a custom-developed decision support system (DSS). This system for evaluating vertical alignments, consists of three modules: a quantitative risk module, a cost-benefit module and a sensitivity module. DSS evaluations show that the number of extra accidents and casualties on the alignments with the steeper grade are limited. As a result, extra damage and accident costs are also limited. From the cost-benefit analysis, it can be concluded that extra costs for traffic accidents and congestion, because of the steeper grade, exceed the saving in construction costs by far. The sensitivity analysis shows variations of the input data have relatively little influence on this outcome. The case of the 2nd Coentunnel shows that state-of-the-art tools enable a quantitative approach to cost-sensitive design of vertical alignments.
Abstract