The effect of road tunnel environment on car following behaviour.

In order to overcome urban space constraints, underground road systems are becoming popular options for cities. Existing literature suggests that accident rates in road tunnels are lower than those in open roads. However, there is a lack of understanding in how the road tunnel environment affects inter-vehicle interactions. In this study, car following data are obtained from traffic video footages of open and tunnel expressways in Singapore. A total of 15,325 car following headways (with car as the follower) are analysed and significant factors affecting headways are found to be speed, and lane. Significant effect of leading vehicle type is only found for tunnel expressway. Headways are generally longer in the tunnel environment. Assessment of collision time measures and safety margins also reveal safer car following behaviour and lower rear-end collision risks in the tunnel expressway. The results are discussed from a behavioural perspective. Overall, the findings show that road tunnels are superior in terms of safety but at reduced traffic capacity .