Traffic signs provide an important means of communicating information to road users and they need to be visible to be effective. The 2003 Manual on Uniform Traffic Control Devices (MUTCD) addresses sign visibility in several sections, including 1A.03, 1A.04, 1A.05, 2A.08, and 2A.22. Visibility is addressed in portions of these sections through factors such as design, placement, operation, maintenance, and uniformity. The concept of visibility encompasses many different considerations and is difficult to quantify as an overall measure. Specific metrics such as conspicuity, legibility, or retroreflectivity are used to represent the various elements that contribute to visibility. Conspicuity is the ability to identify a target (such as a sign) from its surroundings. It is what helps the user to first see a sign. Legibility is the ability to identify the message (content) of the target. It is what helps the user to read the sign. The night-time environment presents many sign visibility challenges. At night, road users cannot see as many visual cues as they can in the day. This places greater reliance on signs and other traffic control devices. To provide night-time sign visibility, most signs are made from retroreflective sheeting. Retroreflectivity is the property of a material to redirect light back toward the originating source. It is what helps make a sign conspicuous and legible. Existing procedures and technologies for measuring sign retroreflectivity provide one, but not the only, metric for quantifying night-time sign visibility. The Federal Highway Administration (FHWA) has focused significant attention on retroreflectivity in recent years, including developing research recommendations for minimum maintained levels of sign retroreflectivity. Sign location and orientation also impact sign visibility. Signs placed outside of the driver’s cone of vision may not be seen by the driver even though they meet other visibility criteria. Likewise, signs behind obstructions (such as a structure or vegetation) may meet some visibility criteria, but can’t be seen by drivers. To provide maximum effectiveness, signs should be designed, placed, and maintained in a manner that is consistent with MUTCD guidelines. This document provides recommendations and general information about minimum maintained retroreflectivity levels and the methods that can be used to maintain sign retroreflectivity. Information contained in this document is intended for policymakers and managers. A separate FHWA document (available in 2004) provides technical details about the various methods to maintain sign retroreflectivity. (Author/publisher)
Samenvatting