Sign Retroreflectivity: Comparing Results of Nighttime Visual Inspections with Application of Minimum Retroreflectivity Values

Abstract

In 1993, FHWA published research recommendations for minimum levels of retroreflectivity for traffic signs. These values were later revised in a 1998 FHWA report. The minimum values were developed as part of a process to add end-of-service life retroreflectivity values to the Manual on Uniform Traffic Control Devices. There have been few or no comparisons between the use of minimum retroreflectivity values to define end-of-service life and the results of typical nighttime visual inspections. Researchers at the Texas Transportation Institute conducted such an evaluation as part of a Texas Department of Transportation (TxDOT) sign workshop. In the evaluation, TxDOT sign crews evaluated 49 signs on an 8-km (5-mi) closed course. Each sign was rated as “acceptable,” “marginal,” or “unacceptable.” Vehicle headlamp illuminance was also measured. The results of the evaluations were compared with the application of the FHWA minimum values. The results show that whereas one sign did not meet the low-speed FHWA minimum values, the average ratings for the TxDOT sign crews indicated that 26 signs were not acceptable. The researchers identified several factors that affected the average sign ratings. These factors included the uniformity of the sign face, the type of sheeting material, and the retroreflectivity. The findings indicate that among the TxDOT sign crews participating in the evaluation, visual nighttime inspections result in a higher failing rate than does the application of the FHWA research recommendations for minimum levels of retroreflectivity. This finding indicates that visual sign inspections should be an important component of assessing the nighttime visibility of traffic signs.