The Stockholm bypass (Förbifart Stockholm) 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. 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 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 driving 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 3D-model of the Stockholm bypass tunnel in the VTI driving simulator III. There was simulated traffic in the main tunnel to improve the realism. Two relatively small gap sizes between vehicles in the main tunnel were used, representing gape sizes that road users can observe on a daily basis when using the E4 motorway through Stockholm. Driving performance (speed, time headway, vehicle position, and distance to tunnel wall) and the test drivers’ experiences of the driving task 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 section. The distance-to-wall results for the Vinsta ramp (0.5 km) with heavy traffic are 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.