Quantitative Risk Analysis (QRA) models have recently been proposed for assessing the societal risk expressed by a frequency / number of fatalities(F/N) curve of road tunnels. For a particular road tunnel, traffic volumeand maximum percentage of vehicles carrying hazmat passing though the road tunnel are two key parameters influencing the societal risk. To evaluateimpact of these key factors on the societal risk, this paper first presents a QRA model based procedure that employs the QRA model to generate F/N curves for all the possible combinations of these two factors. Some combinations of these two factors may result in F/N curves which do not meet a predetermined safety target. This paper thus proposes an excessive risk index of an F/N curve to quantify magnitude of the excessive risk. The two-factor impact analysis can be illustrated by a contour chart based on excessive risk. Finally, a road tunnel case study in Singapore is carried out. It has shown that the maximum tolerable traffic volume rate is 1400 vehs per hour per lane with a maximum acceptable proportion of hazmat transport not more than 2%. Further discussion also shows that higher traffic volume could be acceptable if certain effective operational procedures are taken to reduce the risk of transportation of hazmat.
Abstract