IMPACTS OF PAVEMENT REFLECTANCE ON HEADLAMP ILLUMINANCE FALLING ON A SIGN AND THE RETURNED LUMINANCE

A congressional mandate was set for FHWA to establish minimum retroreflectivity values for traffic signs. The development of minimum retroreflectivity values includes a human factors element as well as an analytical element. The analytical element involves the relationship between the illumination provided to a sign by the vehicle headlamps, the properties of the retroreflective sign sheeting, the geometry of the viewing conditions, and resultant sign luminance returned to the driver. Photometric modeling techniques have been developed and are available to estimate the nighttime performance of traffic signs. However, research has shown that field measurements of illuminance and luminance usually result in higher values than modeling estimates. One explanation for this consistent difference is that the current modeling techniques do not account for factors such as the combination of diffuse and specular pavement reflectance. Furthermore, a review of the literature reveals a void in terms of quantifying the impacts of pavement reflectance from a vehicle headlamp--sign geometry perspective. Consequently, research is being completed at the Texas Transportation Institute to identify the impact of pavement reflectance on headlamp illuminance falling on a sign and the returned sign luminance, as seen from a driver's point of view. This paper will describe the approach used to test pavement reflectance and summarize the results.