Global Objective/Rationale
Nighttime visibility of signs and pavement markings is recognized as essential for highway safety, efficient traffic flow, and driving comfort. The Manual of Uniform Traffic Control Devices, the Traffic Control Device Handbook, Roadway Delineations Handbook, and other design standards are based upon this premise.
The FHWA's Road Departure Program Area is based upon the premise that the first objective is to keep vehicles on the road. This requires effective delineation of the roadway in both day and night conditions. Night visibility can be provided in many ways, but primarily through headlight illumination of traffic control devices (TCD), illumination of the TCD or its legend itself, or by overhead fixed lighting. While all of these means are employed, headlight illumination is the most cost-effective means of providing nighttime visibility. The objective is to provide adequate levels of luminance of a traffic sign or pavement marking to assure that it is first conspicuous to the driver and then legible. Since luminance is hard to measure in the field, the effectiveness of headlight illumination is measured in terms of retroreflectivity.
Retroreflectivity is the property of a material to reflect light back towards its source. All surfaces will reflect light, but in most cases the light is scattered in all directions. Mirrors provide a surface that reflects light in a specular fashion, or at some specific angle from the surface. Retroreflectivity is achieved by using micro-sized glass beads, either enclosed or encapsulated, or by using micro-prisms (cube corner reflectors) into the sign sheeting material. Variations in the technology result in differing levels of retroreflectivity. The higher the retroreflectivty measure, the greater the amount of light returned towards the source. This property has been exploited in materials used for traffic signs and pavement markings to provide nighttime visibility
Figure 1 - Nighttime Crash Trends by Lighting Condition from FARS Data
Since, wearing or aging of the materials, changing conditions of headlights, ambient atmospheric conditions, and other physical factors influence night visibility, there are reasons to use materials with high retroreflectivities. The biggest reason may however, be to meet the needs of the older drivers. A 59-year old driver needs 8 times more light than a 20 year old to see the same object while driving.
