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Driving in tunnels - impact from design and driving environment factors on driving performance


Further information


The Stockholm bypass (Förbifart Stockholm, or FS, in Swedish) project is a new road project that will create a bypass of central Stockholm. The entire project includes motorways, bridges and two tunnels; one of which will be 16.5 km. The Stockholm bypass is the largest infrastructure project in Sweden to date. A high level of road traffic safety is always important and when the road is in a tunnel, and especially in a long tunnel, maintaining the highest possible level of safety is paramount. The present report describes a simulator study on the merging of traffic from entry-ramps into the main tunnel. The entry-ramps in the Stockholm bypass tunnel are planned to let traffic merge into the main tunnel from five specified locations (excluding the main southbound and northbound entrances).


The present study focused upon the specific situation of driving down the entry-ramps and entering (merging) into the main tunnel with a special emphasis on measures of safety and driver performance. A group of 21 test drivers, 11 males and 10 females, participated in the study. They were instructed to drive a series of test scenarios in a 3Dmodel of the Stockholm bypass tunnel in the VTI driving simulator III. There was simulated traffic in the main tunnel to improve the realism for the drivers merging from the entry-ramp tunnel into the main tunnel. The gap sizes between vehicles in the main tunnel were relatively small, two gap sizes were used (1.5 s and 2.5 s). The gap sizes are not intended to represent mean values that allude to total traffic intensity. They are, however, gap sizes that road users will observe on a daily basis when using the E4 motorway through Stockholm. This fact is the rationale for using relatively small gap sizes because they reflect real-life traffic situations. The study design was a within subject design where all test drivers drove all the four included tunnel conditions. Driving performance (speed, time headway, vehicle position, and distance to tunnel wall) and the test drivers' experiences of the driving task (CR10 ratings of four dimensions) were measured.


The main results of the study suggest that the merging zones were too short for some of the drivers to merge comfortably and safely. The merging zones are found at the point where the entry-ramp tunnels merge with the main motorway tunnel and comprise an observation section, an adjustment section and a completion (/taper) section. The distance-to-wall measure (a measure that gauges how much of the entry-ramp remains at the time of merge-completion) for the Vinsta ramp (0.5 km) with heavy traffic 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 remaining before the end of the completion section.

In order to establish the causes and to seek and verify possible solutions to the safety concerns arising from this study, a number of areas need to be explored. Would for 10 ViP publication 2014-3 instance, lengthening the merging zone at Vinsta improve the safety of that entry-ramp? What are the characteristics of an entry-ramp into a motorway tunnel that gives provision for safe and well-working merging? On the other hand, what causes unsafe merging performance? Could it be other aspects apart from the actual length of the merging zones? Moreover, only two of the five entry-ramps have been assessed in this study. What is the status of the remaining entry-ramps from a safety perspective? It is also important to investigate the effects of heavy traffic (buses and trucks) on interaction with other road users and merging.


Project manager: Ruggero Ceci, Swedish Transport Administration (STA), +46 (0)70 560 44 31

Project partners: STA and VTI

(Volvo Technology sub-contractor to STA)


The project is collaborating with the Stockholm By-Pass Tunnel project. The project is co-funded by ViP and STA.

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