Abstract:In the field of traffic safety, research on the operating speed has always been a hot spot and focus. However, most of the researches take road scenes as research objects, and more attention is paid to establishing operating speed prediction models. There are few scholars take extra-long tunnels as research objects, and pay attention to the impact of vertical slope rate on traffic safety. This paper measured 46 sections, running speed of the vehicles in 6 extra-long tunnels, which includes the entrance and exit openings, and the emergency parking zones in the caves, and calculated the average running speed and operating speed of small passenger cars, medium-sized cars and large vehicles. The analysis results show that: 1) to the large-scale vehicle, the sensitive limit slope of the extra-long tunnel is about ±0.75%; 2) It can reduce the speed difference between the operating speed and the average speed of the vehicle, under the condition that the longitudinal slope of the extra-long tunnel is -1.45%~-1.95% and 0.5%, which is beneficial to traffic safety in the extra-long tunnel. This study provides scientific guidance for the design value of the slope of the extra-long tunnel.