Abstract: To investigate driver speed-selection behavior and vehicle-speed responses of different speed-limit schemes in urban tunnels in mountainous cities, a case study is conducted on the Chonggang Tunnel in Chongqing, China. An unmanned aerial vehicle and radar speed gun collect speed data under unified and differential speed-limit schemes. Statistical analyses characterize vehicle-speed distributions and speeding behavior before and after the speed-limit adjustment. The 85th-percentile speed forms the primary reference for optimizing the existing speed-limit scheme. The results show that: (1) Traffic in the Chonggang Tunnel comprises small, medium, and large vehicles, and motorcycles (not included in this survey), among which small vehicles are the main models, the proportions in the two trials are 94.41% and 84.49%. (2) Under the differential speed-limit scheme, the observed speed ranges in the left and right tubes are 40－110 km/h and 30－100 km/h respectively, approximately 10 km/h higher than those observed under the unified speed-limit scheme. (3) At the cross-section located 25 m from the tunnel entrance near the interchange, vehicles exhibited an acceleration-deceleration pattern. After the speed-limit adjustment, vehicles in the left tube change from accelerating before entering the tunnel to decelerating; whereas vehicles in the right tube change from decelerating while exiting the tunnel to accelerating. (4) After the speed limit for medium and large vehicles reduces to 50 km/h, both the 85 th-percentile speed and speed difference decrease at risk locations, including the downhill curve in the left tube, although the overall operating speed across the tunnel increases. (5) Increasing the speed limit for small vehicles reduces the proportion of speeding vehicles by approximately 10%, with most speeding cases involving exceedances of less than 20%. In contrast, speeding increases in frequency among medium and large vehicles after their speed limit is reduced. Because the existing speed limit for medium and large vehicles is found to be excessively low, vehicle-type and direction-specific speed limits are proposed: 80 km/h for small vehicles and 60 km/h for medium and large vehicles in the left tube, and 70 km/h for small vehicles and 60 km/h for medium and large vehicles in the right tube.

Key words: mountainous city, urban traffic, speed-limit scheme, vehicle-speed distribution, speeding behavior, urban road tunnel