Cost considerations and requirements of maintaining traffic sign retroreflectivity
According to the Federal Highway Administration, there are five methods for maintaining traffic sign retroreflectivity:
- Visual nighttime inspection
- Measured retroreflectivity
- Expected sign life
- Blanket replacement
- Control sign
Implementing a cost-effective sign retroreflectivity maintenance program requires more than just consideration of the initial cost. With a visual inspection method, the initial costs are low but there is a need for frequent inspections. On the other hand, measuring sign retroreflectivity provides data to manage and plan cost-effective sign replacement. Furthermore, there is not a need for as frequent inspections as the visual inspection method, which also diminishes the risk exposure of maintenance personnel on the highway. Measured sign retroreflectivity also provides an option to transition to the expected sign life method, which provides the lowest long-term cost.
Visual nighttime sign inspections are only effective for identifying signs with very low retroreflectivity levels (e.g., dead signs), and signs with very high retroreflectivity levels (e.g., new prismatic signs). For the required MUTCD minimum levels, nighttime visual inspections produce poor results (only about 50 percent accurate in a range of about 50 to 125 cd/lx/sq m).
On the other hand, measured sign retroreflectivity provides a numeric result that can be used to directly assess a sign’s condition relative to the MUTCD minimum retroreflectivity levels. Handheld measurements and mobile measurements are both very accurate and repeatable, compared to visual nighttime inspections. Measured sign retroreflectivity also provides the highest level of protection from potential tort claims. However, mobile technology offers several advantages over handheld equipment such as:
- Measurements are made while driving down the highway at highway speeds and therefore no equipment has to be in contact with the sign and no personnel are exposed to the hazards of roadside work.
- Measurements are made at real roadway geometries rather than prescribed geometries that do not always represent the real world.
- Twisted and leaning signs are measured as seen from the roadway perspective and can be easily identified as needing routine maintenance (straightening rather than replacement).
- Images of signs are recorded.
- All signs can be measured, including overhead and difficult-to-reach shoulder mounted signs.
- The entire retroreflective area of the sign is measured rather than a few 1-inch diameter areas. This includes the legends and backgrounds of positive contract signs and provides a more representative result.
When the costs of conducting various types of sign retroreflectivity assessment methods are analyzed, the results revealed that while the visual inspection method produces the lowest initial cost, the long-term cost analyses favored the mobile retroreflectivity measurement method. It also turns out that this method provides the lowest risk of inspector exposure and the highest level of protection from potential tort liability concerns.