Unlighted Overhead Guide Sign Feasibility Study
Authors: Helmut T. Zwahlen, Andrew Russ, and Şahika Vatan
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 evalu
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.
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