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14 the time it takes to perceive and react to the curvature by Plastic mounting with corner-cube lenses (Category C). dropping speed. Surface markings are recognized as sufficient for providing Visibility distances were determined through measure- 2 to 3 seconds of preview time, while longer preview times ments of the optical characteristics of the PRPMs, combined require the use of PRPMs or post-mounted delineators (2). At with data from experiments with subjects. Figure 2-1 shows 60 mph (96 km/h), a preview time of 2 to 3 seconds would visibility distances for various types of delineators, including be equivalent to a driver being able to see 2 to 3 PRPMs the three types above: under different atmospheric visibility ahead at the recommended spacing for tangents (every 80 ft distances, including clear weather (Z = 9.3 miles or 15 km), or 24 m), and 4 to 6 PRPMs ahead at the recommended spac- moderate fog (Z = 0.62 miles or 1 km), and heavy fog (Z = 0.12 ing for curves (every 40 ft or 12 m). miles or 0.2 km); for low-beam headlamps in both new and used condition; and under wet and dry pavement conditions. The line denoted "V85 = 100 km/h" (62 mph) indicates the 2.3.2 Visibility of PRPMs distance necessary to provide 5 seconds of preview time for The visibility of PRPMs depends on aspects of the device, its placement, the vehicle head lighting, the highway geom- etry, and the driver visual capabilities. Drivers detect the NEW 300 presence of a delineator by means of slight differences in Z = 15 km dry wet brightness between the delineator and the road surface. This difference or contrast, C, is defined as 250 200 LT - LB C= (2-7) LB V85 = 100 km/h 150 V85 = 80 km/h Where 100 LT = target luminance and LB = background luminance. 50 Once contrast reaches a certain level, known as the thresh- 0 old contrast, it is just detectable to the viewer. During the 250 day, the visibility of delineators depends only on the contrast Z = 1 km between the delineator and the pavement background. At 200 visibility distance (m) night, visibility depends on the light from headlights as well as on the retroreflectivity of the delineator. Retroreflection 150 V85 = 100 km/h means that the light is reflected back at the same angle at V85 = 80 km/h which it is projected. If light from the headlights were to be 100 perfectly retroreflected, it would not reach the driver's eyes, which are above the headlights. Since retroreflection is imper- 50 fect, some of the light reaches the driver's eyes, increasing the contrast between the delineator and the low-reflectance (pave- 0 ment) background. The higher the percentage of light that is 250 dipped headlamps Z = 0.2 km reflected back to the driver's eye, the greater the contrast and new the further away the delineator will be seen. 200 in practice 150 Device Features V85 = 80 km/h 100 Device design and condition both have strong effects on V85 = 70 km/h visibility distance. With respect to device design, Blaauw and 50 Padmos (27) compared three types of PRPMs that varied in the arrangement and number of lenses: 0 paint thermo- profile profile A B C plast 1 2 raised pavement markers Metallic mounting with three large, biconvex lenses (Category A), Figure 2-1. Predicted visibility distances immediately Plastic mounting with 21 small, biconvex lenses (Cate- after application for all markings on a dry and wet gory B), and pavement (Z = atmospheric visibility distance) (27).

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15 the 85th percentile of the velocity distribution on a rural road 6 2.10 vehicles with a 62-mph (100-km/h) speed limit. As can be seen in Fig- 300 ure 2-1, the PRPMs with corner-cube reflectors (Category C) Z = 15 km dry wet had visibility superior to the other two categories (A and B). 250 Headlights deteriorate over time, causing visibility distances to shorten. This phenomenon can be seen in Figure 2-1, where 200 values that are designated "new" have a calculated visibility based on isocandela diagrams provided by the headlamp man- 150 V85 = 100 km/h ufacturer and values that are designated "in practice" were V85 = 80 km/h used in the experiment. The distances are based on the mea- 100 sured retroreflection coefficients. Requirements for minimum visibility distances are given for rural roads with V85 veloci- 50 ties of 50 and 62 mph (80 and 100 km/h). The requirements for heavy fog (Z = 0.12 miles, or 0.2 km) are lower because 0 of the lower effective speeds on roads that experience foggy 250 conditions (27). Z = 1 km While PRPMs provide better visibility than painted or tape 200 visibility distance (m) lane markings, PRPMs deteriorate more rapidly over time. Figure 2-2 shows visibility distances 22 months after appli- 150 V85 = 100 km/h cation on a highway lane with an AADT of 3,062 veh/day. V85 = 80 km/h As can be seen, visibility distances for in-service devices are 100 reduced by as much as half of that of newly installed devices. However, even under dry conditions, the visibility of PRPMs 50 is still better than the visibility of paint. 0 250 Environmental Conditions Z = 0.2 km dipped headlamps new 200 in practice As shown in Figures 2-1 and 2-2, rain and atmosphere trans- mittance strongly affect delineation visibility. The atmospheric 150 conditions examined include clear weather, moderate fog, and V85 = 80 km/h heavy fog. Reductions in visibility of PRPMs due to rain were 100 on the order of 10 to 20 percent depending on the type of V85 = 70 km/h marker and the environmental conditions. Reductions due to 50 decreased atmosphere transmittance were larger, ranging from 40 to 60 percent. However, the visibility of the PRPMs was 0 still better than the visibility of paint under all conditions, even paint thermo- profile profile A B C after the PRPMs had been in service for 22 months. plast 1 2 raised pavement markers Figure 2-2. Visibility distance 22 months after application of the markings (27). Headlighting Headlight patterns affect delineator visibility. A typical low-beam pattern is shown in Figure 2-3. Headlights are sight of the driver, the more visible the device will be. On aimed to the right and down a few degrees to avoid glare for curves, maximum visibility will be obtained when the PRPM oncoming drivers. This means that more light falls on the face is aligned perpendicular to the tangent of the curve. right side of the road than on the left side, and, with low- Recommended spacings between PRPMs on tangents and beam headlights, delineators on the right will be visible at curves are in the "Roadway Delineation Practices Hand- longer distances than those on the left. Headlights deteriorate book" (2). over time, causing visibility distances to shorten, as can be seen in Figures 2-1 and 2-2. Driver Characteristics Road Geometry Contrast sensitivity. Driver characteristics, mainly con- trast sensitivity, affect delineator visibility. Sensitivity to con- Road geometry affects delineator visibility. The more the trast varies greatly among drivers, even among drivers with face of the delineator is aligned perpendicular to the line of "normal" acuity of 20/20. Because of differences in contrast

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16 Figure 2-3. Isocandela diagram of a typical U.S. low-beam pattern superimposed on a road scene (18). sensitivity, driver detection distances for delineation devices Pietrucha et al. (28) examined the response of drivers in three can vary by a factor of 5 to 1. As drivers age, contrast sensi- age groups (1845, 6574, and 75+) to 25 delineation treatment tivity declines, reducing preview distances available and lead- combinations. Table 2-6 describes 12 treatments. The baseline ing many older drivers to reduce nighttime driving. treatment (Treatment 1) was a 4-in. (100-mm) yellow center- line with a measured coefficient of retroreflected luminance Age. In an FHWA study directed at the needs of older (RL ) of 100 mcd/m2/lux (referred to as an in-service brightness drivers with respect to delineation of horizontal curves, level). Left and right curves were studied with a radius of TABLE 2-6 Details of 12 delineation treatments (28) Treatment Off-Road Edge Centerline Treatment Edgeline Treatment Number Treatment 1 4-in. Yellow Line None None 2 4-in. Yellow Line 4-in. Structured Line None 3 4-in. Yellow Line + Yellow PRPMs None None 4 4-in. Yellow Line + Yellow PRPMs White PRPMs None 5 4-in. Yellow Line None Normal Mount Chevrons 6 4-in. Yellow Line 4-in. White Normal Mount Chevrons 7 4-in. Yellow Line None Standard Flat Posts (Hi-Intensity) 8 4-in. Yellow Line 4-in. White Standard Flat Posts (Hi-Intensity) 9 4-in. Yellow Line None Full Reflection Posts (Hi-Intensity) 10 4-in. Yellow Line None T-Posts (Hi-Intensity) 11 4-in. Yellow Line + Yellow PRPMs None T-Posts (Hi-Intensity) 12 4-in. Yellow Line 4-in. White T-Posts (Engineering)