Safety Evaluation of Wet-Reflective Pavement Markings

Chapter 1. Introduction

Background on Strategy

Although policies vary by jurisdiction, most roadways with any significant volume of traffic have edge lines, center lines, and in the case of multilane roadways, lane lines. These markings provide guidance to drivers on the intended vehicle path. The strategy investigated in this study involves upgrading these existing markings from standard marking materials to wet-reflective markings. Wet-reflective markings are designed to provide an improved level of retroreflectivity during wet road surface conditions. Wet-reflective markings can be applied as a paint, tape, or thermoplastic material.

Glass beads are normally used in pavement markings to reflect light from the headlights back to the driver. They work well when the road surface is dry, but when the surface is wet, the water can act like a mirror, reflecting light in a different direction and often creating glare. New innovative pavement markings include both glass beads and ceramic elements that better reflect light back toward motorists to help them determine locations of driving lanes, edge lines, and merge indicators when they are dry or covered by a thin film of water.⁽²⁾

Background on Study

FHWA established the DCMF program in 2012 to address highway safety research needs for evaluating new and innovative safety strategies (improvements) by developing reliable quantitative estimates of their effectiveness in reducing crashes. The ultimate goal of the DCMF program is to save lives by identifying new safety strategies that effectively reduce crashes and promote those strategies for nationwide implementation by providing measures of their safety effectiveness and B/C ratios through research. State transportations transportation* departments and other transportation agencies need to have objective measures for safety effectiveness and B/C ratios before investing in new strategies for statewide safety improvements. Thirty-eight State transportation departments provide technical feedback on safety improvements to the DCMF program and implement new safety improvements to facilitate evaluations. These States are members of the ELCSI-PFS, which functions under the DCMF program.

The use of wet-reflective pavement markings was selected as a strategy for evaluation as part of this effort.

* Revised 3/12/2019

Literature Review

A literature review found no published research evaluating the effect on crashes of applying wet-reflective markings. Limited research was available on the relationship between retroreflectivity and crashes in general.

Carlson et al. studied the relationship between crashes and the retroreflectivity readings of edge lines, lane lines, and center lines.⁽³⁾ Crash types considered were nighttime crashes occurring outside of intersections or interchanges in the non-winter months of April to October. Wet/ice/snow-involved crashes were also excluded. Geometric and retroreflectivity data from Michigan spanning 2002 to 2008 for two-lane roads and freeways were matched to the crash data. The data were analyzed using negative binomial generalized linear models with blank retroreflectivity readings filled in using average degradation rates and/or information from the closest segment with a reading. For two-lane roads, the effect of yellow line retroreflectivity on nighttime crashes was significant at levels under 150 mcd/m²/lx and showed crashes decreased as retroreflectivity increased. At higher values, there was no indication of further reduction in crashes. The effect of white edge lines also showed a statistically significant relationship with nighttime crashes and a reduction in crashes with increased retroreflectivity that was not dependent on the range of the level of retroreflectivity used in developing the model. For freeways, the results showed decreases in nighttime crashes when yellow or white edge reflectivity increased. For white lane lines, decreases in nighttime crashes were found when the data were limited to segments with readings less than 200 mcd/m²/lx.

Smadi et al. investigated the relationship between the retroreflectivity of pavement markings and nighttime run-off-road and cross center line crashes.⁽⁴⁾ Only rural roadways were considered. Logistic regression modeling was applied to assess the increased probability of a crash when retroreflectivity values were lower. The analysis did not find a correlation between retroreflectivity and crashes except when the level of retroreflectivity was below 200 mcd/m²/lx. Below that level, an increased probability of a crash was found.

Donnell et al. investigated the relationship between pavement marking retroreflectivity and crashes in North Carolina.⁽⁵⁾ First, an artificial neural network model was developed to predict the degradation of retroreflectivity over time. Then, the estimated retroreflectivity was combined with road inventory and crash data to estimate target crashes on a monthly basis using cross-sectional regression models. Target crashes were those nighttime crashes related to visibility, including non-work-zone related, non-alcohol related, dry roadway, no roadway contributing circumstance, ran-off-road fixed-object off road, and sideswipe crashes. The results found that on multilane roads with increased retroreflectivity levels for white edge and lane lines, fewer crashes occurred. Where yellow center lines were used, however, an increase in crashes resulted with higher levels of retroreflectivity. For two-lane roads, a decrease in crashes was found for both white edge lines and yellow center lines at higher levels of retroreflectivity, but these results were not statistically significant at the 90-percent confidence level. The findings also indicate that a measureable impact on crashes requires a significant change in retroreflectivity. For instance, a 50-unit increase in the pavement marking retroreflectivity of white edge lines on multilane roadways reduced the expected nighttime target crash frequency by approximately 18 percent. A 50-unit increase in the pavement marking retroreflectivity of white skip lines on multilane highways decreased the expected nighttime target crash frequency by approximately 10 percent. Similarly, a 50-unit increase in the retroreflectivity of yellow edge lines on multilane highways was related to a 35-percent increase in the expected nighttime target crash frequency. Donnell etal. conducted a literature review of several studies but found no relationship between retroreflectivity levels and crashes.⁽⁵⁾ However, those studies did not account for degradation in retroreflectivity over time.

Bahar et al. developed models of retroreflectivity based on age, color, marker type, climate region, and amount of snow removal.⁽⁶⁾ These models were then used to evaluate the safety impact of retroreflectivity on nighttime non-intersection/interchange crashes using data from California. The results indicated no relationship between nighttime non-intersection/non-interchange crashes and retroreflectivity level. In reviewing the Bahar et al. report, Carlson et al. pointed out that California’s policy was to restripe higher volume roads up to three times a year with paint and every 2 years with thermoplastic markings.⁽³⁾ Consequently, few roadways with significant volumes should reach retroreflectivity levels below 100 mcd/m²/lx, as predicted by the Bahar et al. models applied. Carlson et al. also questioned the grouping of segments that had very low retroreflectivity readings with other segments that had adequate levels of retroreflectivity in the analysis.⁽³⁾