Maintaining high levels 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. In Sweden, the Stockholm Bypass tunnel (FFS), has been scheduled to commence construction in 2013. The tunnel will be approximately 18 km in length. The length of the tunnel is expected to affect the drivers’ experiences pertaining drowsiness, arousal, distraction and feelings of safety and security. The study included 24 participants, 12 men and 12 women, aged 30-45. All of the participants drove two versions of the tunnel in VTI driving simulator; with a decoration design in terms of strings of lights in the ceiling of the tunnel and without any decoration design. The results revealed that 58 per cent of the participants preferred the tunnel with the strings of light in the ceiling. The participants perceived feelings of their driving through the tunnel suggested that the tunnel with the ceiling light design was experienced as being more ”visually cluttered” than the tunnel without the light strings but at the same time it was also experienced as more ”arousing/stimulating”. The negative safety implications of the elaborate interior lighting features would appear to be minimal in terms of distraction and irritation whereas the safety benefits in this particularly long road tunnel, in terms of subjective feelings of visual stimulation is encouraging. Based on the participants’ experiences of the interior design concept of the 18 km long tunnel, having stimulating lighting features in different locations alone the length of the tunnel is recommended.