A popular saying in the light measurement industry is that retroreflectivity is the optical equivalent of the phrase "right back at you". Basically, retroreflectivity is the measurement of efficiency of a highway safety marking to return light in the general direction from which it came. It is simply a ratio of the light visible to the driver compared to the light entering the highway marker. Retroreflective sheeting was first introduced in the 1930's. However, it was not until the 1950's that the enclosed lens retroreflective sheeting became available and retroreflective signs became universally recognized. Since then, technological improvements have led to newer products with increased retroreflectivity and angularity. The types of sheeting material differ with regard to their level of intensity and method of retroreflection (e.g. enclosed,encapsulated or microprismatic materials). Recognition by highway agencies of the importance of retroreflectivity has made the use of retroreflective highway markings nearly universal. According to the "Manual on Uniform Traffic Control Devices (MUTCD)," markings that must be visible at night should be retroreflective unless ambient illumination assures adequate visibility. Because the percentage of well-illuminated roadways is so small, the trend among highway agencies is to make all highway and pavement markings retroreflective.
Over time, retroreflective paints and materials degrade due to the effects of traffic and weather. In addition,we have a large aging driver population that needs brighter signs and pavement markings to maintain its mobility, especially at night. Unfortunately, inadequate and poorly maintained signs and markings are often cited as the contributing factor to nighttime accidents. To assess the effectiveness of safety measures, the Federal Highway Administration (FHWA) and others use the Highway Safety Information System (HSIS), a multi-state database that contains accident information and an inventory of road conditions and markings. This database and others show that while only 25 percent of travel occurs at night, it is then that about 55 percent of the fatal accidents occur. It is not always easy to determine the best time to replace the retroreflected surfaces. If replaced too soon, maintenance costs are increased. If replaced too late, safety and driving comfort are compromised. Thus, strategies have to be developed and implemented to cost effectively maintain minimum nighttime visibility requirements. There are four basic strategies that you can use to maintain a minimum level of highway reflectivity:
- Total Replacement - every sign is replaced after a set interval of time.
- Sign Inspection - all signs are periodically checked, those not up to standards are replaced.
- Sign Management - a computerized database is used to monitor and predict when replacement is needed.
- Sign Inspection combined with Sign Management - the collected field data is stored in a computerized database to more economically manage the minimum nighttime visibility requirements.
Sample sign inspection combined with computerized database and predictive algorithms seem to be the least costly method of maintenance. There are two methods of evaluating traffic sign retroreflectivity under highway conditions. One is measurement using sensors and integrated documentation equipment (retroreflectometer). The other is making "measurements" using human observers. Research has been done to evaluate the accuracy of human observers in making retroreflectivity observations. Observer sign ratings and the sign rating calculated using a retroreflectometer were incorporated into a sign replacement decision model. The individual trained observers only made the correct observation/decision 74% of the time for warning signs and 75% for stop signs. This is a low number where human lives and traffic safety are concerned. In addition to being more accurate, instrument records can be used quantitatively by an agency to assist in defending tort claims.