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HFG MARKINGS Version 1.0 VISIBILITY OF LANE MARKINGS Introduction Visibility of lane markings refers to the ease with which drivers can see and follow longitudinal lane markings. Lane markings are designed for a certain preview time, the amount of time that drivers look ahead on the roadway. This preview time is affected by the distance at which drivers can see markings, which is a function of retroreflectivity and marking width. Different lane marking patterns and colors can have different meanings and regulate different driver actions, such as exiting, lane changing, passing, and maintaining roadway position. For this and other safety reasons, it is important that drivers are able to see and understand lane markings from an appropriate distance. Design Guidelines Factor Guideline Preview Time Absolute minimum preview time = 3 s Recommended preview time = 5 s Marking-Specific Minimum Dark Luminance = 100 mcd/m2/lux Luminance Minimum (adjusted for dirt) Dark Luminance = 121 mcd/m2/lux Marking Width If there is concern about the visibility of the markings, use a 6 or 8 in. marking width instead of the standard 4 in. mcd = millicandela Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data MATHEMATICAL ESTIMATION OF VISIBILITY DISTANCE BASED UPON MARKING RETROREFLECTIVITY AND WIDTH (1) (MODELS ARE FOR YOUNG DRIVERS AND DO NOT CONSIDER GLARE) Visibility distance (D) for longitudinal road markings Visibility distance (D) for a continuous road marking of in high-beam illumination 10 cm width in uniform illumination (simulated daylight) RL (mcd/m2/lux) Where: Where: RL is the coefficient of retroreflected luminance (and C is the contrast ratio between the pavement RL (road) = 15 mcd/m2/lux) marking and the roadway Luminous intensity is constant towards the road L is the luminance in cd/m2 markings (10,000 cd) (Note: Road surface luminance levels in Europe typically range from 0.5 to 2 cd/m2.) 20-2
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HFG MARKINGS Version 1.0 Discussion Preview time: There is some disagreement regarding the minimum amount of preview time that should be provided for drivers. Rumar and Marsh (2) determined through a literature review that a 5-s preview time accommodates proper anticipatory steering behavior, safe steering on roads that are not straight, and the minimum long-range preview time. However, the same review revealed that the Commission Internationale de l'Eclairage (CIE) recommended a lower bound of 3 s for preview time. Schnell and Zwahlen (3) suggest adding an 85th percentile eye-fixation duration of 0.65 s to the 3-s minimum chosen by the CIE to account for the time required for the driver to see and process the marking information. This value is also supported by the COST study, which found that drivers initially had a 2.18-s average preview time, but when the visibility of road markings in the on-road study was increased, the preview times increased to 3.15 s on average (1). Additionally, drivers increased their speed very little to compensate for the increased marking visibility (equivalent to approximately 0.1 s of the time increase) and thus preserved the remainder of the preview time. Therefore, this recommendation is to provide a 5-s preview time when possible, but a 3-s preview time as an absolute minimum. Retroreflectivity: Pavement line retroreflectivity affects the distance from which drivers can view a pavement marking. In a study using subjective observer ratings, Graham, Harrold, and King (4) found that 85% of participants 60 years of age and older rated markings with retroreflectance values of 100 mcd/m2/lux or greater as being adequate or more than adequate when viewed under nighttime conditions. They also calculated a 21% increase in this value (to 121 mcd/m2/lux) to account for occluded light due to dirty windshields and headlights for vehicles that are reasonably maintained. Additionally, more than 90% of the young subjects rated a marking retroreflectance of 93 mcd/m2/lux as adequate or more than adequate for night conditions. In another study utilizing subjective ratings, Ethen and Woltman (5) also found 100 mcd/m2/lux to be the minimum for dark conditions. Note that the luminances that were rated as acceptable were much higher (300 to 400 mcd/m2/lux) in comparison to the minimum values (5). Marking width: The standard width for most longitudinal pavement markings is 4 in. In a survey of state highway agencies, 58% have used markings that are wider than the standard 4-in. marking for centerline, edge line, or lane line applications (6). The data are limited regarding the effectiveness of these markings. However, when surveyed, drivers placed high priority on the quality of pavement markings (6). A variety of studies have shown that when wider than standard pavement markings were used, mean lateral placement was more centered, fewer lane departures on curves were observed, and lanekeeping in low-contrast situations improved (6). Gates and Hawkins (6) concluded that these wider markings show benefits for locations where a higher degree of lane or roadway definition is needed, such as in horizontal curves, roadways with narrow or no shoulders, and construction work zones. Although many of these findings result from a test of one width (either 6 or 8 in.), Gibbons, McElheny, and Edwards (7) found that visibility distance increased for a 6-in. width, but not correspondingly for the 8-in. width. This finding suggests that there may be a threshold where performance does not significantly increase with an increase in line width. Design Issues Problems with glare are more pronounced with the elderly, because optical deficiencies of the eye increase with age. In addition to the temporal visual impairments, glare can cause discomfort and fatigue. In a simulator study with a 4-in. edge line and opposing headlamp glare conditions, subjects aged 65 to 80 required an increase in contrast of 20% to 30% over a younger sample to correctly discern downstream curve direction. To accommodate less capable drivers, the study suggests an increase in stripe brightness of 300% (8). Gates, Chrysler, and Hawkins (9) found that short-range driving performance, including activities such as lane positioning, is more reliant on driver peripheral vision than foveal vision. Wider markings are believed to provide a stronger signal to the driver's peripheral vision over standard width markings, thereby improving driver comfort and short-range performance. Most studies about marking width involve long-range driving tasks such as end detection, which are performed by foveal vision. Cross References None. Key References 1. Commission Internationale de l'Eclairage (1999). COST 331: Requirements for Horizontal Road Marking. Luxembourg: Office for Official Publications of the European Communities . Retrieved from ftp://ftp.cordis.europa.eu/pub/cost-transport/docs/331-en.pdf. 2. Rumar, K., and Marsh, D.K., II (1998). Lane Markings in Night Driving: A Review of Past Research and of Present Situation (UMTRI-98-50). Ann Arbor: University of Michigan Transportation Research Institute. 3. Schnell, T. and Zwahlen, H.T. (1999). Driver preview distances at night based on driver eye scanning recordings as a function of pavement marking retroreflectivities. Transportation Research Record, 1692, 129-141. 4. Graham, J.R., Harrold, J.K., and King, L.E. (1996). Pavement marking retroreflectivity requirements for older drivers. Transportation Research Record, 1529, 65-70. 5. Ethen, J.L., and Woltman, H.L. (1986). Minimum retroreflectance for nighttime visibility of pavement markings. Transportation Research Record , 1093, 43-47. 6. Gates, T., and Hawkins, H.G. (2002). The Use of Wider Longitudinal Pavement Markings (0024-1). College Station: Texas Transportation Institute. 7. Gibbons, R.B., McElheny, M.J., and Edwards, C.J. (2006). Impact of pavement marking width on visibility distance (06-1859.pdf). Proceedings of the Transportation Research Board 85th Annual Meeting [CD-ROM]. 8. FHWA (1997). Synthesis of Human Factors Research on Older Drivers and Highway Safety Volume 2 (FHWA RD-97-095). Retrieved from http://ntl.bts.gov/DOCS/97095/index.html. 9. Gates, T., Chrysler, S., and Hawkins, H.G. (2002). Innovative visibility-based measure of effectiveness from wider longitudinal pavement markings (VIS2002-30). Proceedings of the 16th Biennial Symposium on Visibility and Simulation . Retrieved from http://arrow.win.ecn.uiowa.edu/symposium/DraftPapers/VIS2002-30.pdf. 20-3