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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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6. RECOMMENDATIONS AND CONCLUSIONS
Recommendations for minimum levels of pavement marking retroreflectivity were developed using the results described in the previous section. The recommended levels are shown in table 11. They should apply to MUTCD warranted center line and edge line pavement markings, including lane lines on Interstate highways and freeways, measured under dry conditions in accordance with the 30-m (98.4-ft) geometry described in ASTM E1710. (41) The levels in table 11 apply to both yellow and white pavement markings. The reduction factor recommended for RRPMs assumes that the RRPMs are in good working condition and that at least three of them are visible to nighttime drivers at any point along the road. On two-lane highways with RRPMs along the center line only, the reduction factor applies to both center lines and edge lines. The recommended minimum RL values shown in table 11 are not intended to apply to every combination of geometry, speed, and pavement marking configuration that may be encountered on a roadway. Drivers may require a higher RL in certain situations and engineering judgment should be used to determine if conditions warrant higher RL values. Additional information concerning the basis of these recommendations is described below.
Table 11. Recommended minimum RL values in [mcd/m2/lux ].
* Applies to both yellow and white pavement markings.
A key item under consideration was the pavement surface. The TARVIP analysis showed that the required RL values are higher for an old asphalt pavement surface. As the majority of the roads in the United States are surfaced with old asphalt at any point in time, (7) the recommended minimum retroreflectivity levels are based on aged asphalt surfaces.
Another key item to consider is the vehicle type. Passenger sedans accounted for 57 percent of vehicle miles traveled in the United States in 2003, (42) not including pick-up trucks, vans, and sport- utility vehicles. The TARVIP analysis showed that the required RL values are generally higher for the passenger sedan than the commercial vehicle. The literature shows that the higher the driver eye height (and headlamp height), the lower the needed retroreflectivity. Therefore, the minimum retroreflectivity levels recommended in table 11 are based on a passenger sedan.
Another factor that was considered is pavement marking configuration. The MUTCD includes warrants for center lines and edge lines based on roadway classification, roadway width, and average daily traffic (ADT).(6) The application of longitudinal pavement markings beyond that required by the warrants in the MUTCD is optional. In order to gain some perspective on the presence of edge lines on rural two-lane highways, a recent report from Texas was reviewed which shows that about 40 percent of the rural, two-lane, two-way, state-maintained highway miles in the state have no edge lines. (43)
The recommendations in table 11 are based on different marking configurations. An obvious outcome is that roadways with only a center line need more retroreflectivity tha n roadways marked with edge lines. Previous research has shown similar results in terms of visibility as a function of marking configuration.(24)
The results in table 10 agree that for a constant preview time, required pavement-marking retroreflectivity is sensitive to speed. Therefore, for roadways without RRPMs, the recommendations in table 11 are based on the three roadway speeds investigated in the analyses.The final factor to consider is RRPM presence. The results of the analyses confirm that a discount factor of approximately 45 percent suggested by Molino et al.(29) for a combination of lines with low luminance and RRPMs with medium luminance is reasonable. Therefore, the minimum retroreflectivity recommendations reflect an approximately 45 percent discount factor when RRPMs are deployed and maintained.