SAFETY EFFECTS OF FLUORESCENT YELLOW WARNING SIGNS AT HAZARDOUS SITES By Kimberly A. Eccles Engineer I Bellomo-McGee, Inc. 8601 Georgia Avenue Silver Spring, MD 20910 And Joseph E. Hummer, Ph.D., P.E. Associate Professor Department of Civil Engineering North Carolina State University Raleigh, NC 27695-7908 Tel. 919-515-7733 Fax 919-515-7908 Email [email protected]Submitted to the Transportation Research Board, for presentation at the 80 th Annual Meeting, Washington, DC, January 2001, and subsequent publication in the Transportation Research Record July 2000 Word count: 6080 text + 1250 tables = 7330 total Key words: sign, warning, yellow, fluorescent, safety, collision, hazard
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SAFETY EFFECTS OF FLUORESCENT YELLOW WARNING SIGNS AT HAZARDOUS SITES
By
Kimberly A. Eccles Engineer I
Bellomo-McGee, Inc. 8601 Georgia Avenue
Silver Spring, MD 20910
And
Joseph E. Hummer, Ph.D., P.E. Associate Professor
Department of Civil Engineering North Carolina State University
Submitted to the Transportation Research Board, for presentation at the 80th Annual Meeting, Washington, DC, January 2001, and subsequent publication in the Transportation Research
Events Ran red signal 10 0.2 5 0.1Quick deceleration 4 0.1 1 0
None 4033 99.7 4527 99.9Total 4047 100 4533 100
F Encroachments Stayed in lane 64 57.7 78 67.8Minor yellow 9 8.1 1 0.9Major yellow 1 0.9 0 0Minor white 35 31.5 34 29.6Major white 2 1.8 2 1.7
Total 111 100 115 100
Before period After period
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Table 4. Speed results.
Before Afterperiod, period,
Site Parameter mph mphD2 Mean 42.6 41.2
Standard deviation 4.8 5.550th percentile 43 4285th percentile 47 46
F Mean 36.6 34.6Standard deviation 4.0 4.4
50th percentile 36 3485th percentile 41 39
H Mean 49.2 48.9Standard deviation 4.0 3.9
50th percentile 49 4985th percentile 53 53
I (EB) Mean 47.6 45.6Standard deviation 5.5 4.4
50th percentile 48 4585th percentile 53 50
I (WB) Mean 46.3 45.8Standard deviation 4.5 4.1
50th percentile 46 4685th percentile 51 50
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Table 5. Summary of statistical tests.
Site MOE Change from before to after SignificanceReflect a probableincrease in safety?
2.5% increase in the amount thatmaintained lane
Not at 90%
3.6% decrease in the amount thatencroached on the white edgeline
Not at 90%
0.9% increase in the amount ofvoluntary full stops
Not at 90%
8.7% decrease in amount of non-stopping vehicles
Yes, at 99%
9.3% increase in amount that beganstopping at greatest distance
Yes, at 95%
6.4% decrease in amount that beganstopping at least distance
Yes, at 95%
Traffic Conflicts Decreased from 12 conflicts to 8 Not at 90%
Traffic Events Decreased from 14 events to 6 Yes, at 95%
Mean speed decreased by 1.4 m.p.h Yes, at 95%
1 mph decrease in 50th and 85thpercentile speed
Yes, at 90%
Standard dev. increased by 0.7 mph Yes, at 90%10.1% increase in the amount ofvehicles that maintained lane
Yes, at 90%
5.1% decrease in the amount thatencroached on the white edgeline
Not at 90%
12.2% decrease in the amount thatencroached on the yellow centerline
Yes, at 99%
Standard dev. increased by 0.4 m.p.h. Not at 90%
Mean speed decreased by 0.3 m.p.h Not at 90%
Standard dev. decreased by 0.1 mph Not at 90%
2 m.p.h. decrease in mean speed Yes, at 95%
3 mph decrease in 50th %tile Yes, at 99%
2.8 mph decrease in 85th %tile Yes, at 99%
Standard dev. decreased by 1.1 mph Yes, at 95%
0.5 mph decrease in mean speed Not at 90%
0.9 mph decrease in 85th %tile Not at 90%
Standard dev. decreased by 0.4 mph Not at 90%
ACenterline and
EdgelineEncroachments
No
Possibly, Yes
B
Stop SignObservance
Stopping Distance
Yes
D1
D2Speeds approaching
intersectionNo
Speeds approachingcurve
Centerline andEdgeline
EncroachmentsF
Yes, at 99%
No (WB)
2 m.p.h decrease in mean, 50th, and85th percentile speed
H
Speeds approachingintersection EB
Speeds approachingintersection WB
I
Yes
NoSpeeds approaching
curve
Possibly, yes (EB)
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Tables 3, 4 and 5 show that the installation of fluorescent yellow warning signs appeared
to increase safety at some hazardous sites in this study. One striking finding from the
experiment is that all MOEs except two improved in the after period. The standard deviations of
speed at sites D2 and F were the only two MOEs that declined in the after period, and of these
only the change at D2 was significant at the 90 percent level. Many of the improvements were
statistically significant at the 90, 95, or 99 percent level. The improvements in stop sign
observance and brake light distance at Site B and in vehicles staying in lanes at Site F are
especially striking, as those are strong MOEs. The decline in red light violations at Site D1 is
also important. In all, the improvements at Sites B, D1, F, and I (eastbound) are large enough
and or in strong enough MOEs that it is likely that those sites would have small long-term
reductions in their collision frequencies.
Fluorescent yellow appeared to be most effective at the experiment sites where the
warning signs provide advance information that is not reiterated by other features. For instance,
at site B, the warning signs inform the motorist they are approaching a stop sign. Due to the
geometry of the location, the stop sign itself is not visible until several hundred feet beyond the
warning sign. The warning sign provides information vital to the motorist so that he or she can
prepare for the stop sign. Similar geometry limitations are present at Site D1, Site D2, and Site
F. At Site D1, the view of the traffic signal can be obstructed by a combination of the geometry
and other vehicles. At Site D2, the view of the traffic signal is obstructed by a building until
almost immediately before the intersection. At Site F, the surrounding environment masks the
severity of the curve. At Site B, Site D1, and Site F, the changes attributable to fluorescent
yellow signs are strong enough that they likely increased safety. At Site D2 although the mean,
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50th percentile, and 85th percentile speeds were significantly decreased, the standard deviation
about the mean speed was significantly increased, so the fluorescent yellow signs likely did not
materially increase safety at this site. By contrast, at Sites A and H the warning signs provide
redundant information (the curves were more visible and there were chevrons at Site A) and the
fluorescent yellow signs probably did not improve safety much at those sites.
RECOMMENDATIONS
Based on the results, the researchers provide three categories of recommendations: for use of
fluorescent yellow sign sheeting, for similar studies, and for future research.
Recommendations for Use
The researchers recommend fluorescent yellow warning signs as an inexpensive safety
countermeasure where more driver attention to a hazard is needed. At sites like those examined
in this study, use of fluorescent instead of standard yellow sheeting in warning signs will likely
save a few collisions over the long term. Fluorescent yellow sheeting appears most beneficial
where the roadway geometry or obstructions hide the hazard for which the sign is providing the
only warning. No traffic control device can substitute for removing the hazard, of course.
Although fluorescent diamond grade sheeting costs approximately fifteen percent more
than standard yellow diamond grade sheeting ($5.44 per square foot as opposed to $4.72 per
square foot in April 2000), over the lifetime of the sign the extra cost is not significant compared
to other countermeasures. The estimated cost of installing a 36” standard yellow Diamond
Grade sign in North Carolina is currently $161. The estimated cost of installing a 36”
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fluorescent yellow Diamond Grade warning sign is $178. Sheeting costs are lower for a standard
size (36”) Diamond Grade fluorescent yellow sign than for an oversize (42”) Diamond Grade
standard yellow warning sign ($49 to $58). Making a yellow sign fluorescent is also far less
expensive than installing a flashing beacon above the sign.
Fluorescent yellow sheeting appears to provide a low cost method to increase the safety
of hazardous sites like those tested by increasing the conspicuity of the warning sign. However,
the findings of this study are limited to only seven experimental sites in a single North Carolina
county. More research is suggested before wider application of fluorescent yellow sheeting is
recommended or before agencies should consider requiring fluorescent yellow sheeting.
Recommendations for Similar Studies
The researchers intended to randomly select the experimental sites from a list of candidates.
Although the list of locations received from the NCDOT had fifty-five locations on it, all but
seven had to be excluded based on the site selection criteria. Although at the time the request
was made fifty-five sites seemed large enough, a larger list of locations should be requested for
similar studies in the future.
Recommendations for Future Research
The ultimate measure of effectiveness of a yellow warning sign is the number of collisions it
prevents. The indirect measures employed for this study support that fluorescent yellow warning
signs increase safety at hazardous locations like those tested. However, relating changes in the
indirect measures to actual collision savings is difficult. A collision study would not only help to
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corroborate the findings of this study, but would also help quantify the collision savings.
Additionally, the long-term effects of fluorescent yellow sheeting remain unknown. A large-
scale collision study could also determine if certain types of signs provide more collision savings
when changed to fluorescent than others.
A collision study of fluorescent yellow signs would also help show the magnitude of
collision savings a sign improvement could produce in general. Distressingly few collision
studies of signs are available in the literature—most studies have used indirect measures.
Engineers comparing the safety effects of sign countermeasures to the safety effects of other
types of countermeasures are usually forced to compare “apples to oranges” (collision results to
indirect measure results). Yellow warning signs are abundant, permanent, and (often) placed at
hazardous sites, making them great candidates for learning about the effects of signs on
collisions in general.
ACKNOWLEDGEMENTS
The research was supported by the Southeastern Transportation Center, which is in turn
supported by the USDOT University Transportation Centers Program, and North Carolina State
University. The 3M Company provided fluorescent yellow sheeting for the new signs,
Corrections Enterprises fabricated the new signs, and the NCDOT provided the sites and the
crews to install the new signs. The authors are grateful to all of these entities for their support.
The authors wish to thank particularly Mr. David Burns and Mr. Tim Inglis of 3M; Mr. Chuck
Congleton of Corrections Enterprises; and Mr. John Permar, Mr. Scott Collier, and Mr. Mike
Stout of the NCDOT.
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The views and opinions expressed in this paper are those of the authors and do not
necessarily reflect the views and opinions of any of the entities named above. The authors
assume full responsibility for the accuracy of the data and conclusions presented in this paper.
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