## 3 Results and Discussion
### Effects of RPIDLs on Vehicle Speeds at Entrance of Long Downhill Tunnel  
#### 3.1.1 Effect on Vehicle Speeds at Different Observation Points  
The maximum and minimum speeds of vehicles controlled by RPIDLs with different group lengths are shown in Figures 9 and 10, respectively. Traffic accidents are correlated positively with maximum speed, so decreasing the latter is a good way to reduce the former. From Figure 9, the vehicles controlled by the 120 m RPIDLs had the highest maximum speed at all six observation points. The 30 m RPIDLs had no obvious effect on the maximum speed: that of vehicles controlled by those RPIDLs was almost the same as that of vehicles without RPIDLs. The 60 m RPIDLs gave the lowest maximum speed, meaning that they are the best for controlling the maximum vehicle speed and improving traffic safety in the tunnel.  
However, the vehicle speeds should also not be too low: speeds that are too low test the patience of drivers and reduce traffic efficiency in the tunnel. From Figure 10, the minimum speeds of vehicles controlled by the 30 m, 60 m and 120 m RPIDLs were generally higher than that of vehicles without RPIDLs, indicating that with RPIDLs, drivers adapt to the tunnel environment more quickly and regain their driving confidence. Comparison shows that the 60 m RPIDLs gave the highest minimum speed at points 2, 3, 4, and 6 and so generally offer the best speed control.  
Figure 8: Assessing vehicle speeds controlled by VIDD.  
The differences between the maximum and minimum vehicle speeds are given in Table 1, and the vehicle speeds at different observation points are shown in Figure 11. From Table 1, the difference between the maximum and minimum speeds of vehicles controlled by the 60 m RPLDLs is the lowest among the three different RPLDL group lengths; it is 20.2, 13.4, and 33.3 km/h lower than those of the vehicles without RPLDLs and controlled by the 30 m and 120 m RPLDLs, respectively. Meanwhile, the standard deviation with the 60 m RPLDLs is also lower than those with the 30 m and 120 m RPLDLs; the results show that the vehicles controlled by the 60 m RPLDLs have the lowest speed dispersion. From Figure 11, the average vehicle speed without RPLDLs decreases from observation point 1 to point 3 and then increases from point 3 to point 6. The reason for this is that when vehicles approach the tunnel, their drivers tend to decelerate in response to the changed driving environment; however, after adapting to the driving environment, the drivers tend to accelerate back to the original speed. There is no obvious difference in driving behavior between vehicles controlled by the 30 m RPLDLs and those without RPLDLs. The 60 m RPLDLs delayed the acceleration time, with vehicles accelerating only after passing point 5; this indicates that the 60 m RPLDLs can affect a driver's visual response and actively control the vehicle speed. With the 120 m RPLDLs, the driving behavior exhibits no obvious pattern: the speed fluctuates obviously, and the difference between the maximum and minimum speeds is the highest.  
Figure 10: Minimum speeds of vehicles controlled by RPLDLs with different group lengths.  
Figure 9: Maximum speeds of vehicles controlled by RPLDLs with different group lengths.