Unlighted Overhead Guide Sign Feasibility Study Executive Summary

Agreement Number: 11347

Report Number: OH-2002/001

for copies of this report, go to:

http://www.dot.state.oh.us/divplan/research

or call 614-644-8173

Ohio Department of Transportation

1980 West Broad Street

Columbus OH 43223

Problem

Past and present practice in Ohio is to illuminate overhead guide signs on freeways at night using external luminaires. This practice has enhanced the visibility and legibility of signs made with standard sheeting materials, such as engineer grade (ASTM Type I) and high intensity (Type III).

Newly developed microprismatic materials with higher retroreflectivity are now available and are known as ASTM Types VII, VIII, and IX sheeting materials. These materials have the potential to allow the use of unlighted overhead guide signs in the future. Given the availability of these materials, the continued lighting of highway signs becomes a questionable practice in terms of energy consumption, environmental impact, and cost to the public. Besides the electrical energy consumed, illuminated signs have costs for luminaire installation and maintenance. These costs may be eliminated with the adoption of the practice of using unlighted overhead guide signs.

Objectives

The aim of this study was to compare selected signing materials in certain legend/background combinations to determine if there is adequate conspicuity, legibility, and appearance to allow ODOT to erect and use overhead guide signs on freeways

without lighting at night, and to provide a recommendation to ODOT based on the results. The material combinations compared were beaded Type III legend on beaded Type III background, microprismatic Type IX legend on microprismatic Type IX background, microprismatic Type VII legend on beaded Type III background, and microprismatic Type IX legend on beaded Type III background.

These combinations were compared with and without overhead sign illumination using an expert panel evaluation and a photometric evaluation.

Description

The study had two parts. The first part was a nighttime evaluation of a series of signs erected on US30 near Mansfield by 12 ODOT engineers and technicians riding in a 2002 Dodge Caravan. Signs were rated on the basis of conspicuity, legibility, and overall appearance. The Type IX on beaded Type III and Type VII on beaded Type III signs were rated the highest overall. Type IX on Type IX performed slightly worse, while beaded Type III on beaded Type III performed significantly worse.

The second part was a series of nighttime photometric measurements made at 3M’s test track in Minnesota with a Prometric CCD camera from a 1984 Peterbilt truck, 2002 Chrysler minivan, and 2002 Toyota Camry at selected distances from the sign bridge ranging from 100 ft (30.5 m) to 1000 ft (305 m). The parameters measured were the legend luminance and the luminance contrast ratio between the legend and the background. Both legend luminance and luminance contrast ratio are important factors in sign legibility. The results showed that the Type VII on beaded Type III combination had the highest legend luminance and highest luminance contrast ratio when all distances were considered. The Type IX on beaded Type III combination performed better at distances of 200 ft (61 m) and performed as well as the Type VII on beaded Type III at 400 ft (122 m).

The beaded Type III on beaded Type III signs almost always performed the worst, and the Type IX on Type IX sign was usually in between.

Conclusions & Recommendations

The practice of lighted overhead signs can be discontinued if either white Type VII or Type IX legends are used on green beaded Type III backgrounds. Microprismatic Type III materials were not evaluated in this study. The use of microprismatic Type III may be suitable provided the luminance values are comparable to those of the beaded Type III materials evaluated in this study. The use of microprismatic Type III materials having significantly higher luminance values would most likely reduce the contrast ratio and thus could reduce the legibility of the signs they are used on.

Using microprismatic Type IX or Type VII legends on beaded Type III backgrounds on unlighted overhead guide signs should not result in any detrimental information acquisition and safety effects to the majority of the driving public. The evaluator results are based upon a group of traffic engineers and technicians ranging in age from 27 to 48 years (average 38) riding in a 2002 Dodge Caravan. The photometric results are based on measurements made from a 2002 Chrysler Town & Country minivan (similar to the Caravan), a 2002 Toyota Camry, and a 1984 Peterbilt truck. While these results and conclusions are expected to apply to the majority of drivers, special cases such as older drivers (over 65 years old) or newer large trucks with lower headlamp profiles (and greater observation angles) than the Peterbilt tested may not see as much benefit from these signs as the majority of motorists. Further research is recommended to look into these issues.

Implementation Potential

We recommend that ODOT prepare a statewide implementation plan and schedule to discontinue the practice of providing and maintaining luminaires for overhead signs and replace step by step all overhead signs in the State with microprismatic sheeting legends (Type VII or Type IX) on beaded Type III background sheeting. The high contrast ratios observed in this study for Type VII on beaded Type III and Type IX on beaded Type III may not occur if a microprismatic Type III sheeting is used for the background, since the microprismatic Type III material is more retroreflective and has a higher luminance than the beaded materials tested.

The implementation plan should include all overhead signs. If there are special cases where for example due to geometric conditions the overhead signs can only be read at relatively short approach distances (less than about 400 feet (122 m)) using a white microprismatic Type IX legend on a green beaded Type III background can be specified.

The change of practice from lighted to unlighted overhead signs with white microprismatic legends on green beaded Type III background will have a number of benefits including the elimination of the luminaire installation costs, the electricity requirements at overhead signs, the electricity costs, the maintenance and associated traffic control costs, and the wasted illumination towards the night sky (“light pollution”). These benefits by far outweigh any potential negative effects.