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Publication Number: FHWA-HRT-07-059
Date: October 2007
Updates to Research on Recommended Minimum Levels for Pavement Marking Retroreflectivity to Meet Driver Night Visibility Needs
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The results of the TARVIP runs are shown in table 9. Each cell contains the required RL for a unique set of vehicle type, vehicle speed, pavement surface, and pavement marking configurations. The minimum RL values for the scenarios that included RRPMs are also shown in table 9. Table 10 shows the results of the TARVIP runs that were generated to evaluate the sensitivity of RL to preview time. These runs were made by varying the preview time and speed while keeping the vehicle (passenger sedan), pavement surface (old asphalt), and pavement marking configuration (yellow center line with white edge lines) constant.
Table 9. Minimum retroreflectivity levels in [mcd/m^2/lx].
Table 10. Required RL values for TARVIP scenarios with varying preview time in [mcd/m^2/lx].
The results in table 9 show good agreement with recent research. For instance, in many cases, the retroreflectivity levels associated with the Freightliner vehicle are less than they are for a passenger vehicle. Gibbons et al., as well as Rumar et al., discovered the same finding in their recent works.(17, 19) On a fully marked roadway, this visibility advantage for the Freightliner was slight, and there was even an advantage for the passenger sedan at lower speeds. In such cases, the height advantage provided by a larger vehicle does not overcome the larger observation angle in a large vehicle. However, in scenarios with the center line only configuration, the Freightliner required RL values ranging from 2 percent to 53 percent less than those required for the passenger sedan. This wide range of values is due to the previously discussed interaction between speed and vehicle type.
The results in table 9 show that the required RL increases as the speed of the vehicle increases. This is in agreement with previous minimum pavement-marking retroreflectivity recommendations (1, 7, 9) that roadways with higher speed limits should have pavement markings with higher retroreflectivity. This is expected, as higher speeds require longer detection distances for the same preview time. In other words, longer detection distances are needed when driving at higher speeds in order to perceive and react to the information provided by the pavement markings.
The results in table 9 also correlate well with previous research that shows that old concrete should provide better visibility than old asphalt.(16) This advantage ranged from nearly negligible at higher speeds to approximately 20 percent at lower speeds. This was also expected as at higher speeds, the preview distances are longer, resulting in larger entrance angles. At large entrance angles, the retroreflectivity values of old asphalt and old concrete measured by Schnell et al. converge.(16)
As expected, the fully marked road scenarios produced much lower required RL values than the center line only scenarios. For the fully marked road scenario (noted as YCL-WEL in table 9), the detection distance used to generate the retroreflectivity levels is based almost entirely on the visibility of the solid white edge line. For this configuration, TARVIP calculates the detection distance for the white edge line and then multiplies by a factor, which varies with geometry, to add additional detection distance due to the presence of the yellow center line, as shown by Zwahlen and Schnell in their previous research.(24) Therefore, the required RL of a white edge line is slightly reduced by the presence of a yellow center line. On the other hand, the addition of solid white edge lines provides a 66 percent reduction in the required RL values for a dashed yellow center line and a dashed white lane line at 64.4 km/h (40 mi/h) and an 85 percent reduction at 112.7 km/h (70 mi/h). The reason that solid white edge lines provide for greater reduction in the required luminance of center lines and lane lines at high speeds may be deduced from the distribution of headlight illuminance on the road surface, presented in figure 4. At greater distances from the vehicle, the difference in the headlamp illuminance on the right edge line to that incident on other lines becomes proportionally greater, increasing the visibility advantage that the solid white edge lines offer to drivers.
Also, when RRPMs are present (and in good working order so that at least three are in view at any time), the required RL values decrease substantially, ranging from 18 to 34 mcd/m²/lux for fully marked roads and 31 to 48 mcd/m²/lux for center line only roads. The results indicate that, at most, RL values no greater than 55 mcd/m²/lux are necessary for the driver to determine the color and configuration of pavement markings. These results also concur with the Molino et al. work, which recommended retroreflectivity discount factors when RRPM are present.(29, 31) The TARVIP results were compared to that research in order to recommend minimum values for pavement marking retroreflectivity when RRPMs are deployed.The results shown in table 10 indicated that required RL is highly sensitive to preview time, especially at higher speeds. Increasing either speed or required preview time increases the detection distance. For a vehicle speed of 112.7 km/h (70 mi/h) and a preview time of 4.0 seconds, this equates to a detection distance of 125.3 m (411 ft) , thus the relatively high required RL of 735 mcd/m²/lux. These results show that selecting a reasonable preview time is critical when determining minimum pavement marking retroreflectivity.