An Analysis of the Factors Influencing the Retroreflectivity Performance of In-Service Road Traffic Signs  
[PERSON]  
1 School of Information and Engineering, Dalarma University, 781 70 Borlange, Sweden; [EMAIL_ADDRESS] (H.  
European standards [7] regulate the color (chromaticity) of road traffic signs based on the CIE color space that was created by the International Commission on Illumination. Permissible colors are indicated by color regions or color boxes in the chromaticity diagram shown in Figure 1. The two chromaticity coordinates, X and Y, for retroreflective sheeting determine a location in the CIE chromaticity diagram which represents a certain color. Accepted colors must be within the color boxes to ensure that road traffic signs have the right color and are suitable for the intended purpose. The color is no longer accepted when it deteriorates beyond these chromaticity limits (color boxes). The coefficient of retroreflection can be measured using retroreflectometers, while spectrophotometers are used for measuring colors. In the field, measurements can be recorded using either handheld or mobile equipment. Currently, handheld devices are most frequently used for measuring sign retroreflectivity and colors, although this approach needs to address safety challenges and the high cost of data collection. Handheld instruments are relatively expensive, must be in direct contact with the sign, and data collection must take place under special weather conditions. Maintenance workers who use these instruments must stand close to the road sign for a relatively long time and they may need to block the traffic, in particular, on motorways or when using a lift. Although mobile devices have several advantages over handheld devices, mobile equipment is much more expensive and has lower accuracy compared to handheld devices. To summarize, the methods used to measure the retroreflection and colors of in-use road traffic signs are costly, time-consuming, need to be performed under special weather conditions, and involve an accident risk for maintenance staff. Therefore, many countries, including Sweden, do not have a systematic program for the inventory of the performance of road traffic signs. At the same time, it is very important to specify when a sign reaches the end of its service life and must be replaced. This study aims to find a safe, easy, inexpensive, and accurate method that can be used by road authorities to analyze the performance and deterioration of retroreflective road traffic signs mounted on roads. Two datasets were used to fit models to predict the retroreflectivity of road traffic signs. The proposed method is to use linear and logarithmic regression models to predict retroreflectivity without needing to use measuring instruments. Previous studies have suggested prediction models based on two features (sheeting type and color). Most of these studies focused on age as the only predictor for retroreflectivity performance, with [PERSON] [9], who considered color and observation angle, in addition to age and sheeting type, being an exception. Therefore, the accuracy of the models in these studies was low. Figure 1: CIE color space with color boxes according to SS-EN 12899-1. This study proposed regression models with multiple predictors to identify (statistically) significant factors that affect retroreflectivity. Furthermore, efforts have been made to find and evaluate models that include only the predictors that can be collected without the need for any expensive measuring instruments. For this reason, the predictors included were the age of the road traffic sign, its GPS position, and the direction of the sign, as well as the color and class of the retroreflective sheeting. One other important issue in this paper was investigating the significance of the chromaticity, luminance factor, and GPS positions on the retroreflectivity of road traffic signs. These factors were not previously investigated and their effects on retroreflectivity were not included in the state of the art.