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Minimum Retroreflectivity Levels for Overhead Guide Signs and Street-Name Signs

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CHAPTER 7. INITIAL RECOMMENDATIONS

Using the results of the data analysis described in chapter 6, the final runs of the TTI MR model were made. These runs resulted in a total of five tables that included the factors considered in this research. The sensitivity analyses demonstrated that there were three particular factors that could not be comfortably narrowed down. Therefore, for the convenience of field use, the tables were consolidated.

One of the first items considered was the accommodation level. As explained in the sensitivity analyses, the accommodation levels used herein represent drivers age 55 and older. Because of this, the accommodation levels do not represent the entire population of nighttime drivers. However, data are not available that detail the age distribution of nighttime drivers. With the assumptions that 75 percent of nighttime drivers are younger than age 55 and that younger drivers need less luminance than older drivers to read traffic signs, an accommodation level of 50 percent for the data collected herein actually translates to an accommodation level of 75 + [(50/100) × 25] = 87.5 percent for the population of nighttime drivers. This correlates well with the 85^th percentile accommodation level for which the previous MR levels were designed. Therefore, the consolidation efforts are based on an accommodation level of 50 percent for older drivers.

Another item considered for consolidation was the sign position relative to the vehicle position. For overhead signs, the MR levels were chosen to represent a sign positioned above the left adjacent lane of the travel lanes. It was assumed that low-speed, post-mounted street-name signs are mostly found on local, two-lane roads. For high-speed, post-mounted street-name signs, it was assumed that the driver turning right was in the appropriate lane and used the two-lane road levels. However, for the driver turning left on a high-speed roadway, it was assumed that the road is either a two-lane highway with shoulders or a four-lane arterial/collector without shoulders, both cases providing substantial offset, justifying the use of the multilane levels. Finally, for overhead street-name signs, the MR levels were chosen to represent a sign positioned directly above the travel lane. It was assumed that most drivers realize that they are in the vicinity of where they need to turn and are therefore in the appropriate lane.

Other consolidation efforts included rounding the MR levels for overhead and street-name signs to the nearest integer divisible by five, again for convenience in field applications. Once the rounding off was completed, an effort was made to consider the feasibility of consolidating the various types of retroreflectivity sheeting. In general, the MR requirements for beaded sheeting increase slightly, but consistently, as the sheeting type is increased from types I through III. Therefore, for beaded sheeting, the levels for type II sheeting were used to represent all beaded sheeting (with the few exceptions indicated in the following tables). For microprismatic materials, the trends are less uniform and less consistent. Therefore, consolidation among microprismatic materials was not conducted.

MR levels for the green backgrounds were obtained by calculating the percentage difference between the retroreflectivity of new unweathered white sheeting and the minimum levels as determined through this research for each type of sheeting. The percentage difference was then applied to the retroreflectivity of new unweathered green sheeting. The value was then rounded to the nearest integer divisible by five. This allows the legend and the background of signs made of the same sheeting to degrade at the same rate, allowing agencies to schedule sign replacement activities based on the degradation of one single material. Table 32 shows the values used for new unweathered sheeting.

Table 32. Average R_A of New Unweathered Sheeting

The MR levels presented are meant to be applied to typical roadway sections. They would need to be adjusted for areas with complex backgrounds, roads with a high predominance of large trucks, or frequent severe road curvature; however, there has not been a sufficient amount of research conducted to determine the extent of these adjustments. The MR levels are shown in cd/lx/m² and represent the standard measurement geometry of an observation angle of 0.2° and an entrance angle of -4.0°.

Table 33 presents the initial recommendations for overhead guide signs, table 34 presents the initial recommendations for post-mounted street-name sign levels, and table 35 presents the initial recommendations for overhead street-name levels. The following chapter summarizes additional research that resulted in recommendations that superseded those in tables 33 through 35.

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