Why frequent traffic accidents at highway tunnel exit? – An experimental analysis of the slack effect.

[Display omitted] • There is a slack effect at the exit of highway tunnels. • Slack effect leads to decrease in driver's attention and abnormal vehicle speed. • Accumulation of negative driving emotions in tunnel triggers slack effect. • Complex traffic scenarios should be avoided at the exit of extra-long tunnels. This study analyzes the causes of frequent traffic accidents at the exit areas of highway tunnels. Vehicle experiments were conducted to collect data on the light environment, vehicle speed, and driver eye movement. The distribution patterns of the illumination, vehicle speed, and gaze location occupancy were analyzed, and highway tunnels of different lengths were compared. The tunnel exit area is divided into the transition, exit, and departure zones. It was observed that there was an obvious white-hole effect at the exit zone, with drastic changes in the light environment. The presence of abnormal acceleration behavior in the transition zone and decreased attention in the departure zone demonstrate the existence of psychological and physiological slack in drivers. Additionally, the tunnel length positively correlates to the slack effect, which is stronger in extra-long tunnels than in the short and medium tunnels. Multiple control scenarios were established and driving simulation experiments were conducted to further analyze the impact of the slack effect on the traffic safety. The experimental results demonstrated that the slack effect of the extra-long tunnels exhibited an evident lag which was still present after leaving the tunnel exit. The average vehicle speed and trajectory offset were larger and more discrete in the post-tunnel zone than on an ordinary open road. The speed of the curve in the post-tunnel zone is 5.1 % higher than that of the curve in the ordinary road, and the maximum trajectory offset is 54.3 % higher. Thus, this study reveals the slack effect at tunnel exits and explains the abnormal phenomenon of relatively good traffic environments but high traffic accident rates in the transition and departure zones of tunnel exits. These results present considerable practical significance for the safe operation of highway tunnels, particularly extra-long tunnels. [ABSTRACT FROM AUTHOR]