This report is an investigation of the current methodology used to evaluate traffic sign retroreflectivity under actual highway conditions. The report consists of three parts: a literature survey, a questionnaire, and the training and analysis of human observers to rate traffic sign retroreflectivity.
The literature survey and questionnaire concluded that human subjectivity judgment is almost exclusively used to evaluate traffic sign retroreflectivity. Instruments are not used for large inventories because of cost and computer-based sign management systems are in the early stages of development and not used. The questionnaire also revealed that few states have any policy for sign replacement.
The main objective of the research was to assess the accuracy of using human observers to evaluate traffic sign retroreflectivity. Observers were trained to rate warning and stop sign retroreflectivity in two experiments. After the training the observers evaluated signs on two highway courses. The observer sign ratings and the sign rating calculated using a retroreflectometer were incorporated into a decision model to replace or not replace a sign based on the sign condition and environment. The individual observers made correct decisions on 74 percent of the warning signs and 75 percent of the stop signs.
The literature survey and questionnaire concluded that at present there is no method of sign review other than the trained observer that is suitable for large sign inventories. The experiments have demonstrated that a trained observer is a valuable part of a sign maintenance program. Agencies will have to continue to rely on observers' judgments for some time to come.