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Publication Number: FHWA-HRT-04-139
Date: December 2005
Enhanced Night Visibility Series, Volume VIII: Phase II—Study 6: Detection of Pavement Markings During Nighttime Driving in Clear Weather
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This study is part of the Enhanced Night Visibility (ENV) project sponsored by the Federal Highway Administration. This report summarizes an investigation into the visibility of pavement markings in different vehicle lighting conditions. Of particular interest in this study is the effect of different vehicle vision enhancement systems (VESs) on the visibility of various pavement marking materials. For example, some of the pavement marking materials were fluorescent, and some of the vehicle lighting included an ultraviolet (UV–A) component.
This research evaluated the effects of 3 pavement marking materials and 11 VES technologies across three age groups of drivers: young (aged 18 to 25), middle-aged (aged 40 to 50), and older (aged 60 and over). The research focused on measuring the pavement marking visibility for each combination of pavement marking material and VES.
The research was conducted on a special research facility roadway under controlled conditions. The pavement marking visibility was assessed by evaluating the distances at which the participants were able to see the beginning and the end of a series of pavement markings in each experimental road section.
Pavement markings provide essential tracking and guidance information to drivers at night. In addition, pavement markings provide essential reference points so that drivers can maintain proper vehicle position.(1) For example, Fitzpatrick, Lance, and Lienau implemented lane drop pavement markings that indicated when a lane terminated on a highway exit.(2) The researchers found that driver behavior was modified such that with the addition of the markings, drivers were inclined to be more proactive and make less erratic maneuvers compared to a baseline of no such markings.
For pavement markings to be effective, it is suggested that they provide the driver with a 3.65-s preview time.(3) To attain that level of conspicuity, the pavement markings must provide adequate contrast with the pavement and draw the attention of the driver.(4) Contrast (C) is dependent on the luminance or lightness of both the marking (LT) and the pavement (LB) as calculated in the equation in figure 1.(5)
Overall, materials differ in the amount of contrast they provide. Turner, Nitzburg, and Knoblauch investigated differences between pavement marking materials with a specific focus on new paint, worn paint, new thermoplastic, and fluorescent thermoplastic.(6) The fluorescent thermoplastic was found to be the most visible when viewed with both halogen and UV–A headlamps. Other pavement marking materials were not as visible as the fluorescent thermoplastic.
Pavement marking visibility is affected by the retroreflectivity of the material. The coefficient of retroreflected luminance is a measure of the amount of headlamp illumination that the pavement marking material reflects back to the driver, resulting in the level of marking luminance (LT). (Note that in keeping with common usage, the terms "reflective" and "reflectance" may be used in place of "retroreflective" and "retroreflectance.") Zwahlen and Schnell observed that if the reflectance of pavement markings was high, drivers modified their visual search.(3) Drivers exhibited increased longitudinal visual search and greater fixation durations. Increased viewing distances allow a potential increase in safety by providing drivers with longer viewing times of the upcoming roadway; however, these potential safety benefits also depend on vehicle speed.
When compared to the visibility from a stationary position, one researcher found a 40 percent decline in pavement marking visibility while driving at 24 km/h, or 15 mi/h.(7) Jacobs et al. argued that higher speeds would create an even greater decline in visibility, thus increasing the need for highly visible pavement markings. Therefore, determining the distance at which pavement markings are visible to a driver when a vehicle is moving is an important aspect to the research on the performance of alternative pavement markings.
To measure the effectiveness of pavement marking materials, it is necessary to consider both vehicle speed and headlamp systems. For example, Zwahlen and Schnell measured the effect of different headlamps on the visibility of roadway markings.(3) The researchers found that highly reflective pavement marking materials paired with low-beam headlamps compensated for medium reflective pavement marking materials paired with high-beam headlamps. The authors concluded that the reflectivity of the pavement markings was considered more important in pavement marking detection than the headlamp configurations; however, the researchers evaluated only two types of headlamps, suggesting that other types of headlamp or VES configurations may be beneficial when identifying pavement markings.
VESs serve two purposes in low ambient illumination conditions—to illuminate pavement markings and illuminate obstacles.(8) Many headlamp technologies have been developed over the past 80 years to assist drivers with detecting and recognizing obstacles and pavement markings. The following paragraphs summarize the technologies evaluated with the pavement marking materials in this investigation. Headlamp specifications appear in ENV Volume XVII, Characterization of Experimental Vision Enhancement Systems.
The tungsten-halogen (halogen) headlamp is currently the most common headlamp available for vehicles in the United Stated and Europe, so the low-beam setting of halogen headlamps (HLB) was used as the baseline condition for the ENV studies.
The high-beam setting of a halogen headlamp (HHB) typically provides drivers with an increased area of illumination. The increased area is achieved through increasing the output of the lamp and also raising the angle of output. The result is that light is projected farther down the roadway, providing greater visual distance.
High intensity discharge headlamps (HID) use a compact metal-halide arc lamp with high luminous efficiency and output, which make them good candidates for vehicular applications.(9) These types of headlamps are becoming standard on some vehicles; however, there has been little direct research measuring the effects of HID headlamps on pavement marking visibility.
UV–A radiation, with wavelengths between 320 and 400 nanometers, is outside of the visible light spectrum. Researchers have determined that, with a combination of appropriate filters and minimized stationary exposure, UV–A headlamps do not pose a safety threat.(10) UV–A headlamps have shown potential for increasing visibility when illuminating objects with fluorescent properties. Mahach, Knoblauch, Simmons, Nitzburg, and Tignor implemented both dynamic and static testing to determine the enhanced visibility of fluorescent pavement markings when viewed with a UV–A headlamp.(11) Overall, a significant improvement was found with the UV–A headlamp configurations when compared to the non-UV–A headlamp setups.
High output halogen lamps are being evaluated as potential replacements for standard tungsten-halogen lamps in headlamp applications. For this investigation, the HOH conditions used prototype lamps to provide higher light output at the same power consumption, with an expectation of increased visibility.
When reviewing visibility issues concerned with both VES technologies and an assortment of pavement marking materials, it was found that driver age is also an important factor. Currently, the percentage of older drivers continuing to drive is increasing as the population grows older.(12) In 2002 there were 19.9 million older licensed drivers, which represented 10 percent of all drivers.(13) When driving at night, older drivers require increased levels of illumination, highly visible pavement markings, and reduced exposure to glare.(3,12,14)
As people age, their vision naturally declines for various reasons. Older drivers exhibit both reduced detection ability and reduced contrast sensitivity.(14,15) The latter may affect older drivers' abilities to detect pavement markings at night. Zwahlen and Schnell compared the detection distances of pavement markings for both young and old drivers.(16) The older drivers were found to have a 55 percent decrease in detection distances from the detection distances of younger drivers. The decrease in detection distance with age may place older drivers at a large disadvantage for navigating the roadways safely at night.
With various age-related factors, all drivers are likely to benefit from increased visibility at night provided by different VESs and enhanced pavement marking materials. In an effort to investigate the effect of these technologies on visibility, the current research study evaluated 11 different VESs in conjunction with 3 types of pavement marking materials.