Using Driving Simulators in Road Design–A Road Safety Study of Merging Traffic in Tunnels

The present study describes a simulator study on the merging of traffic from entry-ramp tunnels into the main Stockholm bypass tunnel. The present study focuses on two of the five junctions with connections at Lovö and Vinsta. Three research questions were formulated relating to 1) Is there a difference in the subjective and/or objective measures between the two different entry-ramp tunnels (Lovö, 1.5 km and Vinsta, 0.5 km)? 2) Is driver performance when merging affected by the drivers’ experience when entering the main tunnel from the entry-ramp tunnel? 3) Does traffic intensity and its subsequent effect on the gap size between vehicles influence the frequency and character of hazardous situations such as late merging? Twenty-one participants completed the study. The main results of the reported study suggest that the merging zones were too short for some of the drivers in order to merge comfortably and safely. The merging zones are found at the point where the entry-ramp tunnel merges with the main motorway tunnel. For the Vinsta (0.5 km) merging zone with heavy traffic the distance-to-wall measure (the measure that gauges how much of the merging zone remains at the time of merge-completion) is particularly concerning from a road traffic safety perspective because more than 25% of the drivers completed the merging manoeuvre with less than two seconds of Time Headway (THW) remaining before the end of the completion section. Two seconds of travel before the ending of the tapered completion section of the merging zone was considered to be the bare minimum in terms of safe driving and safe merging. Two seconds equates to 44.4 m when travelling at 80 km/h. The completion section of the merging zone also tapers to the width of a car (approx. 2 m) 40 m from the end of the merging zone.