During the last ten years, dry road marking retro reflectivity has been measured by the use of mobile equipment. However, due to a growing interest in the performance of wet road markings, the use of road markings with a profiled surface is common in many European countries today. When measuring this type of marking with the use of mobile equipment in wet weather conditions, one problem arises: The measured retro reflectivity will be affected by splash from the wheels of the measurement vehicle, which can be avoided only by driving extremely slowly. Therefore, in order to carry out measurement at the speed of surrounding traffic, measurements must be accomplished on dry road markings.
The goal of the present study is to find an equation which describes the relationship between, on one hand, the wet road marking performance in night-time driving and, on the other hand, the performance in dry condition. The independent variables in this equation must be possible to measure at the speed of the surrounding traffic. This is important, not only for the safety of the operator, but also for traffic safety in general.
The work on finding a new texture factor and a better prediction model has been organized as follows:
- project management. (The Danish Road Directorate)
- theoretical considerations. (Johnson Consulting)
- development of a computer programme calculating the texture factor. (Johnson Consulting)
- field measurements. (Ramböll and VTI)
- statistical analysis. (VTI)
- 6. documentation. (VTI).
The result shows that the fundamental hypothesis can be accepted:
The texture factor is a better predictor of the retro reflectivity of a wet road marking than MPD is. The average difference between measured and predicted values was found being 5.0 mcd/m2 /lx, which is an improvement from 5.6 mcd/m2 /lx when using MPD. However, even if the improvement is significant, it was expected to be larger.
The reason for this relatively small improvement is probably the contribution from micro beads on some road markings, but not on others. One hypothesis is that if the surface of the profile is almost vertical, the water will run off and the beads appear, thus contributing to the retro reflectivity.
Therefore, the improvement of the model would probably be much larger if an indicator for contributing micro beads was also included. An attempt to do so was done, however without success, partly explained by too little data to split the analysis into two subsets; one with contributing beads and one without.
The conclusion of this study is that an important step towards a better prediction model for wet road marking retro reflectivity was taken. However, collection of more data and the introduction of an indicator for contribution from glass beads would probably improve the model even more.