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2 Executive Summary Background and Objectives Longitudinal pavement markings are found on nearly all freeways and highways in the United States, and previous research has emphasized the importance of quantifying the impact of different pavement marking material types on safety. While important, quantifying safety of pavement marking and marker of material types has remained elusive. This study takes a unique approach compared to previous research, and instead focuses upon quantifying the relationship between retroreflectivity and safety over time, independent of marking or marker material type. Study Methodology This study examined the safety effect of retroreflectivity of longitudinal pavement markings and markers over time on non-intersection locations during non-daylight conditions. The National Transportation Product Evaluation Program (NTPEP) is a service provided by the American Association of State Highway and Transportation Officials (AASHTO) which collects data and evaluate pavement markings and markers (among other products) using a formal and detailed work plan. For this study, NTPEP data were assembled into a database and used to derive mathematical models of retroreflectivity performance as a function of age, color, marking material type or marker type, climate region, and amount of snow removal. As a result of this modeling, a significant contribution of this study is the generation the retroreflectivity performance models as a function of various factors, which has never previously been achieved using other datasets. Those models were used to estimate the retroreflectivity of pavement markings and markers on state-maintained freeways and highways in California for 1992-1994 and 1997-2002, covering over 5,000 miles of road segments. An innovative study approach was developed which solves for multipliers that represent the change in the expected number of crashes as a function of retroreflectivity. Safety effect multipliers were solved for yellow and white pavement markings separately and in combination, and for pavement markers for different road types and crash severity, using the retroreflectivity models and Californiaâs data of over 118,000 non-intersection, non-daylight (night, dawn, and dusk) recorded crashes.
3 Discussion of Results The study has produced retroreflectivity models for epoxy, methyl methacrylate, permanent tape, solvent, thermoplastic and waterborne paint for both white and yellow pavement markings. Retroreflectivity models for markers are a function of their type (snowplowable or non- snowplowable). Both markings and markers are modeled as a function of climate region, and the amount of snow removal. Look-up tables based upon the retroreflectivity models in Appendix A may be useful to jurisdictions seeking estimates of their pavement marking and marker retroreflectivity, or for comparing the performance of new products to the average performance of a particular material type. The analysis methodology used in this study solved for multipliers representing the safety effect for different retroreflectivity ranges (bin ranges). The analysis methodology was tested thoroughly with simulated data and it proved that it was able to correctly identify safety effects. The analysis methodology was tested again through sensitivity analysis using the California data set. The approach used in this study was found to be reliable and straightforward to implement and is recommended for safety treatments which change over time. The advantages of this approach were that it allowed for maximum inclusion of historical data and did not have the same sampling problems of traditional before-after studies. In order to correctly analyze the data as a time-series, it was necessary to separate out the monthly seasonal effect from the cyclic pattern of pavement marking and marker installation. Seasonal multipliers were developed for the three road types using all state data for each one of the road types but were not divided by climate regions, thus producing an average seasonal effect. The seasonal effect multipliers showed higher crash counts in January, November, and December which provided support for the validity of the analysis methodology. For pavement markings and markers, the safety difference between high retroreflectivity and low retroreflectivity markings during non- daylight conditions and on non-intersection locations was found to be approximately zero, for all roads that are maintained at the level implemented by California. Our study provides a level of certainty that builds upon previous research such as Lee et al. (1), Migletz et al. (2), and Cottrell and Hanson (3) which were unable to identify any relationship between retroreflectivity and nighttime crashes.
4 In conclusion What appears to be important is that markings are present and visible to drivers, but what is less important with respect to safety is whether the markings have high retroreflectivity or relatively low retroreflectivity. One hypothesis is that drivers compensate by reducing their speed under lower visibility conditions, and maintain higher speeds under higher visibility conditions. Therefore, any effect of the level of brightness of pavement markings or markers may be minimized by driver adaptation to road conditions. In other words, the best estimate of the joint effect of retroreflectivity and driver adaptation is approximately zero for non-intersection road segments during non-daylight conditions. In summary, this study found that there is no safety benefit of higher retroreflectivity for longitudinal markings on non-intersection locations during non-daylight conditions for roads that are maintained at the level implemented in Californiaâs state highways. Californiaâs level of maintenance appears to be frequent with pavement markings being installed on higher volume highways up to three times a year with waterborne paint, or every two years with thermoplastic markings. The findings of this research study allow agencies to recognize that resources to increase the retroreflectivity of longitudinal markings, beyond normal maintenance activities, will not be cost- effective and that those resources could instead be allocated towards other safety measures. This report recommends the following issues be considered for future research: calibration of California retroreflectivity data, study replication in other states, examining the effect of longitudinal markings and markers retroreflectivity on intersection related crashes, examining the effect of marking and marker retroreflectivity on crashes at curves, examining the benefit of pavement marking and marker management systems, retroreflectivity effects on traffic operations, and the human factors of marker and marking visibility.
