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Road Safety Performance Associated with Improved Traffic Signal Design and Increased Signal Conspicuity

2.0 Hypothesis

The conspicuity of traffic signal backboards is improved through the application of highly retro-reflective tape around the outside border. The hypothesis is to test that the increase in conspicuity will have measurable road safety benefits.

The Manual of Uniform Traffic Control Devices for Canada states that traffic signal backboards (backplates) are used to improve the visibility (conspicuity) of traffic control signals both by making the signal head stand out from its surroundings, and by helping to prevent confusion due to distracting features in the background (TAC, 1998). It further recommends that backboards should be used on all primary signal heads. The U.S. Manual of Uniform Traffic Control Devices is more general in that it states that a signal backplate for target value enhancement should be used on signal faces viewed against a bright sky or bright or confusing backgrounds (FHWA, 2000). It further states that the use of backplates enhances the contrast between the traffic signals and their surroundings for both day and night conditions, which is also helpful to elderly drivers. However, neither document outlines any requirements for the sheeting of the backboard.

Furthermore, both manuals state that the colour conspicuity of signs must be maintained under all lighting conditions or otherwise illuminated. Hence many jurisdictions provide sheeting with retro-reflective properties on their signs to meet this requirement and thereby avoiding the need for illumination. Therefore, it is reasonable to assume that in order to improve the conspicuity of signal backboards the boards should be fitted with retro-reflective sheeting. Also, since signal backboards are located overhead, they are disadvantaged with respect to vehicle headlights, aimed to illuminate the roadway directly in front of the driver and not overhead signs. As such, the use of highly reflective sheeting, such as prismatic sheeting (e.g., 3M-diamond grade VIP) is necessary.

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