IMSA Journal Feature Article
RESEARCH RECAP: ARE YELLOW TRAFFIC SIGNALS BRIGHTER THAN THEY NEED TO BE?
John D. Bullough, Lighting Research Center, Rensselaer Polytechnic Institute
Published In LD+A, December 2002
What do we need traffic signals for?
Most North Americans love to drive, and one of the more ubiquitous forms
of lighting on our roads are traffic signals. Among the advantages of these
lighting devices, according to the Millennium Edition of the U.S. Manual
on Uniform Traffic Control Devices (MUTCD, online at mutcd.fhwa.dot.gov)
are orderly traffic flow, increased traffic capacity, and reduced frequency
of crashes. Arguably, the reduction of crashes is paramount, because crashes
can result in the loss of human life. For this reason, the MUTCD refers to
important standards about the color and the required luminous intensities
of traffic signals, so that they can be detected quickly and accurately as
What do the standards say?
In North America, these standards1,2 give the minimum luminous intensities
required for 8- and 12-inch signals of each of the three colors (red, green
and yellow). Most interestingly, green signals are required to have a luminous
intensity that is 2.0 times that of the red signal, while yellow signals
require 4.6 times the luminous intensity of the red signal. These differences
diverge from practice in Europe and Japan, where all three signal colors
are required to have the same luminous intensity.
These differences lead to certain questions: Do yellow and green traffic
signals in North America need to be brighter than the red signal? In particular,
do yellow traffic signals, which have nearly five times the luminous intensity
of red signals, need to be as bright as they are? This last question is particularly
relevant because of the advent of traffic signals using light-emitting diodes
(LEDs) instead of incandescent lamps. Red LED signals are becoming almost
commonplace throughout North America, with green LED signals following closely
behind. Generally, transportation agencies consider the installation of red
and green LEDs to be very successful (they use 80 percent less energy than
their incandescent counterparts, and are warranted to last much longer).
However, no standards-complying yellow LED traffic signal currently exists
on the market. Meeting the higher luminous intensity standard with yellow
LEDs results in a product that uses too many LEDs to be cost-effective, and
in problems with heat dissipation and maintaining light output.
Why does yellow matter?
One could argue that the lack of a standards-complying yellow LED signal
is unimportant. Many agencies are converting to LED traffic signals because
of energy and maintenance savings, but the yellow signal is only on for a
few seconds during each signal cycle, which means yellow LEDs would not significantly
affect the overall life-cycle cost. This is no doubt the case for many retrofit
situations, but it is also true that many intersections convert to a flashing
mode late at night, where the yellow signal is flashed continuously in one
direction and the red signal in another. Here, potential energy and maintenance
savings from yellow LED signals could be meaningful. They might become even
more meaningful in those instances where battery backup is provided in case
the electricity supply is interrupted; using LEDs in flashing mode at these
times might give work crews a few extra hours to handle potential emergency
What is the basis for the higher luminous intensity of yellow (and green)
signals, anyway? Unfortunately there doesn't appear to be any readily available
documentation, but a hint is found when we consider the relative transmittance
of the colored glasses used in incandescent signals. Green glass lets about
twice as much light through as red glass; yellow glass four to five times
as much. And since the red signal is arguably the most critical in terms
of life safety, it might have been perceived by early standards makers as
most important to "get red right" and then to scale the luminous intensities
of the other two colors by the increase in transmittance afforded by green
and yellow glass. (Of course, this remains pure speculation.)
What does the research say?
The Lighting Research Center at Rensselaer Polytechnic Institute recently
completed research that can help standards makers grappling with differences
among various international specifications.3 Against a bright background
that simulated daytime viewing, subjects were asked to detect simulated incandescent
and LED traffic signals of varying luminous intensities and colors. If the
signal was not detected within one second, it was considered a missed signal.
In this way, the reaction times and percentage of missed signals could be
gathered. Thousands of trials were collected systematically in this study.
Of interest, there were no differences in visual response between incandescent
and LED signals having the same color and luminous intensity. A red LED signal
performed just as well as a red incandescent signal. There were, however,
significant differences among the three signal colors. For the same intensity,
red signals consistently resulted in shorter reaction times and fewer missed
signals than yellow and green signals.
These findings suggest that for traffic signals in Europe and Japan, which
are specified with equal luminous intensities, yellow and green signals result
in longer reaction times and in a greater percentage of misses than red signals.
Is such an approach appropriate? Certainly, there does not appear to be any
controversy abroad associated with traffic signals of different colors having
the same luminous intensity. Still, one could argue that cultural differences
among drivers in North America, Europe and Japan make differences in traffic
signal intensities desirable in North America. Again, looking at the results
from the aforementioned study can shed some additional light.
As described above, red LED traffic signals meeting the North American standards
have been widely used in what seems to be a successful manner. Since these
signals seem to be successful, their resulting reaction times and percentage
of missed signals are probably acceptable in practice. We also know that
yellow and green signals having the same luminous intensity as red signals
will result in longer reaction times and a greater percentage of misses,
which might be considered unacceptable in North America (although this is
clearly not unacceptable in Europe and Japan). Using the results of the Lighting
Research Center's study, one can determine how much higher yellow and green
signals must be in order to have the same reaction time and the same percentage
of misses as a red, standards-complying signal.
Using an equivalent reaction time as the criterion, the luminous intensity
multiplication factors are 2.0 for the yellow signal and 2.6 for the green
signal. Using an equivalent percentage of missed signals as the criterion,
the luminous intensity multiplication factors are 1.4 for the yellow signal
and 2.8 for the green signal. In other words a yellow signal that is twice
the intensity of a standards-compliant red signal will result in a reaction
time and a percentage of missed signals no worse than the complying red signal.
If, and only if, this is a reasonable criterion for the performance of a
yellow traffic signal, then the existing standards that require yellow signals
to have a luminous intensity that is 4.6 times that of the red signal are
clearly much higher than needed. As an aside, reducing the luminous intensity
of the yellow signal will also help to make them less uncomfortable to view
at night. Yellow and green signals meeting current standards can often be
considered uncomfortably bright; red signals are almost always not.4
What about the green signal? The results above indicate that a green signal
would need to be nearly three times higher in luminous intensity than the
red signal to perform as well as a complying red signal in terms of reaction
time and missed signals. This is a pretty large increase over what is currently
required by standards. Should standards therefore increase the luminous intensity
required of green signals? It seems unlikely. The type of response that is
required of a green signal (start, or continue moving) is very different
than that required of the red and yellow signals (stop!). There do not appear
to be problems with current green traffic signals in North America, which
are "only" twice the luminous intensity of red signals. It is therefore unlikely
that requiring green signals to have the same reaction time and percentage
of missed signals as a red signal, is a reasonable requirement. Furthermore,
increasing the luminous intensity of green signals would make them more uncomfortable
at night than they already appear to be.
Nonetheless, one could argue that it is also an unreasonable criterion for
the yellow signal, too. Perhaps it is even more important to respond quickly
and accurately to the yellow signal than to the red signal, since after all,
it is a precursor to the red signal. If this is the case, the data from the
study described above can assist in identifying just how much higher the
luminous intensity should be. Other data too will be important in answering
fundamental questions about appropriate luminous intensities, especially
for drivers with color deficiencies, and older drivers. The results from
the studies described here, as well as from previous studies and from a soon-to-be-published
study commissioned by the National Cooperative Highway Research Program,
will be especially helpful as requirements for traffic signals are revisited.
It could be argued that even considering a reduction in the luminous intensity
of traffic signals is irresponsible. After all, any reduction in luminous
intensity will result in longer reaction times and a greater percentage of
missed signals. Whether this reduction in visual performance is meaningful,
is a more difficult question to answer. In addition, concerns about energy
use and costs of maintenance nationwide have prompted some agencies to begin
using non-complying yellow LED signals, regardless of the lack of standards-complying
yellow LED products. These issues make it ever more important that standards
makers consider what visual responses are required for each signal color
and how a given luminous intensity provides the required visual response.
Without careful, yet timely, consideration, the examples of "going-it-alone"
that have already begun will grow while standards-making bodies will grow
less relevant. Such precedents will surely affect organizations like the
IESNA, which develops standards and recommendations for many applications.
Standards makers should act proactively now rather than reacting later to
these inevitable questions.
1. Institute of Transportation Engineers. Vehicle Traffic Control Signal
Heads: A Standard of the Institute of Transportation Engineers. ITE, Washington,
2. Institute of Transportation Engineers. Vehicle Traffic Control Signal
Heads - Part 2: Light Emitting Diode Traffic Control Signal Modules. ITE,
Washington, DC, 1998.
3. Bullough, J. D., P. R. Boyce, A. Bierman, K. M. Conway, K. Huang, C. P.
O'Rourke, C. M. Hunter and A. Nakata. Response to simulated traffic signals
using light-emitting diode and incandescent sources. Transportation Research
Record 1724. TRB, Washington, DC, 2000.
4. Bullough, J. D. P. R. Boyce, A. Bierman, C. M. Hunter, K. M. Conway, A.
Nakata and M. G. Figueiro. Traffic signal luminance and visual discomfort
at night. 80th Annual Transportation Research Board Meeting. TRB, Washington,
IMSA Journal Feature Article