Baltimore Regional Transportation
Board
2nd Baltimore Regional Traffic Signal Forum
New ITE Standards
for
Traffic Signal Lights
Carl K. Andersen
Turner-Fairbank Highway Research Center
McLean, Virginia
14 December 2005
Federal Highway Administration
Department of Transportation
U.S. Requirements for Traffic Signal
Lights
• 23CFR655 Subpart F—Traffic Control
Devices on Federal-Aid and Other Streets
and Highways:
655.601: The Manual on Uniform Traffic
Control Devices for Streets and Highways,
(MUTCD), 2003 Edition, is incorporated by
reference.
Federal Highway Administration
Department of Transportation
2003 Edition MUTCD
• Section 4D.18, Design,
Illumination, and Color of Signal
Sections:
Standard—Except for the
requirements of this section, the
requirements of the “Standards
for Vehicle Traffic Control
Signal Heads” shall be met.
http://mutcd.fhwa.dot.gov/
Federal Highway Administration
Department of Transportation
2003 Edition MUTCD
• Section 4D.18, Design,
Illumination, and Color of Signal
Sections:
Guidance—The intensity and
distribution of light for each
illuminated signal lens should
conform to the current
“Standards for Vehicle Traffic
Control Signal Heads” and
“Traffic Signal Lamps.”
Federal Highway Administration
Department of Transportation
Equipment and Material Standards
of the ITE
• Vehicle Traffic Control Signal
Heads (VTCSH):
Provide a guide for preparation
of specifications for vehicle
traffic control signal heads,
representing the requirements for
signaling equipment.
Not intended to impose
restrictions……
http://www.ite.org/
Federal Highway Administration
Department of Transportation
Equipment and Material Standards
of the ITE
VTCSH approved in April 1985.
VTCSH Part 2: LED Modules
(Interim) effective 17 June 1998.
VTCSH Part 3: LED Vehicle
Arrow Traffic Signal Modules
adopted March 2004.
VTCSH LED Circular Signal
Supplement adopted 27 June
2005. (VTCSH-LED)
Federal Highway Administration
Department of Transportation
ITE Joint Industry and Traffic
Engineering Council Committee
(LED Signals Committee)
Balanced membership—Manufacturers and Users.
•
•
•
•
•
•
•
•
•
Nathaniel S. Behura, Transportation & Energy Solutions, Inc. (Chair);
Seth Chalmers, Chalmers Engineering Consultant;
Andrew Lipman, Next Generation Lighting;
Henri R. Arcand, Consultant (GELcore);
Gary R. Durgin, Dialight Corp.;
David C. Edwards, Intertek Testing Services NA, Inc.;
Ken Morge, North Carolina Department of Transportation;
Carl K. Andersen, FHWA;
Siva Narla, Staff Liaison, ITE.
Federal Highway Administration
Department of Transportation
Why Change?
(was a new standard really needed?)
• The ITE VTCSH was written for signal lights using
incandescent lamps. It included requirements that
would insure hardware uniformity, which have
minimal impact on incandescent systems, but may
greatly impact LED signal lights.
• VTCSH Part 2 was drafted as interim guidance,
when development of high-output LEDs was at its
infancy. Advances in LED technology must be
considered in order to realize cost and energy
savings.
Federal Highway Administration
Department of Transportation
What was desired of a new standard?
• Minimum changes from existing standard (ITE)
–
–
–
Don’t change what has historically worked,
Minimize confusion by driving public,
Maintain existing quality assurance.
• Conformity with International standards
– Improve normalization across borders;
• Allow for comprehension by all drivers,
• Minimize differences in purchase requirements.
• Meet the needs of drivers (human performance
requirements)
Federal Highway Administration
Department of Transportation
Developing a New Standard
• Based on an evaluation of the ITE VTCSH
against the research results and needs of
drivers, it was determined that a revision
to the standard was in order.
• The new standard specifies requirements
for LED circular signals. However,
consideration should be given to drafting
an entirely new ITE VTCSH that is
technology independent.
• The new ITE standard for LED Circular
Vehicle Traffic Control Signals was
adopted on 27 June 2005.
Federal Highway Administration
Department of Transportation
ITE VTCSH-LED
The New ITE LightEmitting Diode Traffic
Signal Specifications—
A Guide for Purchasers
by Nathaniel S. Behura
ITE Journal – November
2005 (pp 38-40)
Federal Highway Administration
Department of Transportation
What are the new requirements?
• VTCSH-LED establishes new standards for LED
signals, including:
– Color;
– Luminous intensity and distribution;
– Physical requirements; and
– Electrical requirements.
Federal Highway Administration
Department of Transportation
What is NOT included in
VTCSH-LED?
• VTCSH-LED is a performance specification to
assist in the purchase of new signal modules.
Consequently, it does not address such factors as:
– Signal placement;
– Replacement/cleaning schedule;
– Monitoring light output degradation; and
– Warranty issues.
Federal Highway Administration
Department of Transportation
What are some of the critical changes?
• Luminous intensity requirements over full range
of environmental temperatures.
• Maximum intensity limitations and requirement
for luminance uniformity.
• Testing under duty cycle (yellow only)
• Low voltage turn-off.
• Failed state impedance.
• Turn-on and Turn-off times.
• Use of “ITE Compliant” label—Qualification!
Federal Highway Administration
Department of Transportation
Luminous intensity & luminance
• Intensity ratio for red, yellow and green
(R:Y:G) has changed to value of 1:2.5:1.3
Maintained Intensity Requirements
at –2.5 deg (V) and 0 deg (H)
Color
200mm
300mm
Red
165 cd
365 cd
Yellow
410 cd
910 cd
Green
215 cd
475 cd
Federal Highway Administration
Department of Transportation
Luminous intensity & luminance
300 mm Red LED Signal
400
350
250
200
-22.5
150
-15
100
0
-7.5
7.5
50
15
22.5
-25
-20
-22.5
-27.5
-30
Vertical Angle (deg)
-12.5
-15
-17.5
30
-5
-7.5
-10
5
2.5
0
-2.5
0
Horizontal Angle (deg)
-30
22.5
20
17.5
15
12.5
10
7.5
Luminous Intensity (cd)
300
• Intensity profile specified in
a continuous curve in space,
and not at discrete angle
pairs.
• Limits set on maximum
luminous intensity to reduce
glare and driver discomfort.
• A maximum permissible
ratio established for
luminance values across
emitting surface (1-inch
spots)
Federal Highway Administration
Department of Transportation
Test per usage
Normalized LED Output vs Time
• Red & green LED
signal lights tested
after 1 hour of
continuous operation.
• Yellow LED signal
lights operated at
12.5% duty cycle for
1 hour prior to tests.
1.05
Green
1.00
LED output (normalized)
0.95
Red
0.90
y = -0.0193Ln(x) + 1.0612
R2 = 0.9751
0.85
0.80
Yellow
0.75
y = -0.0723Ln(x) + 1.2011
R2 = 0.9983
0.70
0.65
0.60
0
60
120
180
240
300
360
420
480
540
600
660
720
Time (sec)
Federal Highway Administration
Department of Transportation
New electrical requirements
• Low-voltage turnoff—intended operation is with
input voltage between 80-135 VAC. Requirement that
there be NO VISIBLE illumination when applied
voltage < 35 VAC.
• Failed-state impedance requirement—in the event of
catastrophic failure of the signal, the module presents
a resistance of at least 250 kΩ across the input power
leads.
Federal Highway Administration
Department of Transportation
Turn-on & turn-off times
Rise Times of Sources
1.5
Normalized Intensity
LED
Incandescent
1
0.5
0
0
50
100
150
200
250
Time (msec)
300
350
400
450
• LEDs have very
rapid rise and fall
times, but power
supply may impact
response.
• Desire all signal in
an intersection to
respond similarly.
Federal Highway Administration
Department of Transportation
The “ITE Compliant” label
• Extensive qualification testing required before a
manufacturer can use the ITE compliant label.
• Design qualification testing required on new module
designs, when a major design change is implemented
or after every 5 years that a design is in service.
• Production tests and inspections incorporate a
simplified set of critical tests.
• Customers always have the ability to require more
comprehensive quality assurance testing.
Federal Highway Administration
Department of Transportation
Conclusion
• VTCSH-LED is a comprehensive PERFORMANCE
specification.
• Agencies are encouraged to use contracting rules to
establish desired warranties…however, remember
benefit:cost ratio.
• It is important that agencies establish appropriate
systematic monitoring and maintenance activities.
• LED technology continues to evolve and improve,
but the benefits in energy savings and reduced
maintenance are already available.
Federal Highway Administration
Department of Transportation
Background
• The following slides provide background information
on the process followed in development of the ITE
Specification for Vehicle Traffic Control Signal
Heads—Light Emitting Diode (LED) Circular Signal
Supplement (VTCSH-LED), dated June 27, 2005.
Federal Highway Administration
Department of Transportation
The process of developing a
human-performance based standard
for direct-emitting signals
• First, clearly identify the purpose of a traffic
signal light.
• Second, evaluate prior research for applicability
to that purpose, especially in light of new
technologies.
• Finally, compare existing standards to driver
needs to determine the requirement for change.
Federal Highway Administration
Department of Transportation
The purpose of a traffic signal light
• Provide a conspicuous, readily understandable
and unambiguous signal regarding right-of-way
at roadway intersections and other potential
conflict areas.
• The intent is to promote public safety.
• As a secondary consideration, a traffic signal
light should provide for efficient traffic flow
and reduce congestion.
Federal Highway Administration
Department of Transportation
The challenge of meeting the
purpose of a traffic signal light
• To meet its purpose, a traffic signal light must
be conspicuous and readily understandable to a
wide range of drivers, including:
– Younger drivers, who will have less experience and
may not watch for signal changes,
– Older drivers, whose vision may have deteriorated
to the point that the signal is not as apparent, and
– Drivers with color-vision deficiencies (CVD).
Federal Highway Administration
Department of Transportation
Prior Research
CIE Publication No. 48 (1980)
• CIE Publication No 48 (1980) provides
information on issues that directly influence the
road user’s perception of road traffic signals. The
document:
– Reviews general terms related to the visual
effectiveness of light signals: attention, perception,
vision, visibility, and traffic engineering.
– Reviews the findings of investigations on traffic signal
requirements.
http://www.cie-usnc.org/
Federal Highway Administration
Department of Transportation
CIE Publication No. 48
Required characteristics of a signal light
• The specific characteristics of a traffic signal light that
impact on the effectiveness of the light signal are the
luminous intensity and color, as projected towards the
observer. Thus, the luminous intensity profile, or beam
distribution, and color are the primary performance
characteristics.
• Factors that impact on the performance of the traffic signal
light are the background luminance, position of the signal
light in the visual field, and the visual capabilities of the
observer.
Federal Highway Administration
Department of Transportation
CIE Publication No. 48
Factors impacting signal performance
• Traffic engineering factors also impact signal performance.
These factors include the distance at which the signal
needs to be detected, the directions in which signals are
needed, and the intersection layout and traffic situation.
– In general, a minimum visibility distance of 100m for
traffic signal lights in built-up areas has been accepted.
• A discussion of traffic signal placement is beyond this
presentation, but is discussed in detail in the ITE Traffic
Control Devices Handbook.
Federal Highway Administration
Department of Transportation
CIE Publication No. 48
Measures of success
• The various studies that are reviewed in CIE Publication
No. 48 did not use the typical psychophysical
measurement of threshold as indicating adequacy of a
signal. Rather, the various experiments evaluated not only
the percentage of correct responses, but also the reaction
time needed to achieve those responses.
• Success was defined at the level at which increases in
signal intensity resulted in little or no improvement to
reaction times.
Federal Highway Administration
Department of Transportation
Review of prior research
Luminous intensity requirements
• The two primary factors that determine the
luminous intensity requirement of a signal light
are:
– The luminance of the background (LB), and
– The distance (d) from which the signal light is to be
seen.
Federal Highway Administration
Department of Transportation
Review of prior research
Luminous intensity requirements
• Adrian, 1963; Jainski & Schmidt-Clausen, 1967;
Cole & Brown, 1968; and Fisher, 1971, and others
indicatee that there is a linear relationship between
the background luminance (LB) and the required
luminance of a signal light (LS) of a fixed size.
LS
LB
= C1
• This allows results of experiments conducted
under various conditions be be compared!
Federal Highway Administration
Department of Transportation
Review of prior research
Luminous intensity requirements
• Cole & Brown (1968), using a simulated driving task, found that the
response of the observer was dependent only on the amount of light
reaching the eye from the signal. Fisher (1974) found that, at the
desired detection distance, signals are not typically on the line of
motion of the vehicle and, thus, are not initially detected in the fovea.
As such, spatial summation areas are larger and signal size is less
important. This results in a simplifying assumption that the optimal
luminous intensity of a traffic signal light is independent of size:
LS ω = C2
where ω is the solid angle subtended by the signal light
Federal Highway Administration
Department of Transportation
Review of prior research
Luminous intensity requirements
• Since the subtense of a signal light is equal to the area (A)
of the signal divided by the square of the distance (d) to
the signal and the luminous intensity is the luminance
times the area (for a signal with a uniform luminance):
ω=A
then:
d
&
2
I
d
2
I = LS A
= C2
Federal Highway Administration
Department of Transportation
Review of prior research
Luminous intensity requirements
• Combining these concepts results in a simplified equation
for the luminous intensity necessary for a signal light to be
seen at a prescribed distance (Id):
I d = Cd LB
2
Federal Highway Administration
Department of Transportation
Review of prior research
Luminous intensity requirements
• As mentioned earlier, a standard detection distance of
100m was considered adequate for most signal light
applications. In addition, a sky luminance of 10,000 cd/m2
was established as a “standard” condition. Systematic
evaluations by Cole & Brown (1966, 1968) and Boissin &
Pagès (1964) indicated that a red signal light of 200 cd,
viewed under standard conditions, will be detected quickly
and with certainty.
−6
I d = 2 × 10 d LB(cd)
2
Federal Highway Administration
Department of Transportation
Review of prior research
Luminous intensity requirements
Sensitivity of the human eye
drops as detection of a visual
stimulus is moved from the
fovea to the periphery. Under
photopic conditions, only the
cone detector system is
functioning, and there is a
rapid drop off in the number
of cones outside the fovea.
Federal Highway Administration
Department of Transportation
Density of Rods and Cones versus Perimetric Angle
Federal Highway Administration
Department of Transportation
Review of prior research
Luminous intensity requirements
• Evaluations by Fisher (1969, 1971) indicated that the
value of 200 cd for a red signal light, viewed under
standard conditions, was needed at an offset angle from
the driver’s line of site of 3 degrees. Fisher also found that
as the angle increased from 3 degrees, the luminous
intensity requirements also increased, with the following
relationship:
( 3)
Iθ = I θ
1.33
Federal Highway Administration
Department of Transportation
Review of prior research
Luminous intensity requirements
Combining the results of all of the research yields
the “Fisher Equation”:
I d ,θ
( )
= 2 × 10 θ
3
−6
1.33
2
d LB (cd )
• Id,θ is the required luminous intensity (cd),
• θ is the angle from the driver’s line of sight (deg),
• LB is the background luminance (cd/m2).
Federal Highway Administration
Department of Transportation
Review of prior research
Effects of color on intensity requirements
• The “Fisher Equation” provides the luminous
intensity requirements for a RED, 200mm (8 in.)
traffic signal light, viewed under standard
conditions in daytime.
• Numerous studies indicated that the luminous
intensity of GREEN and YELLOW signal lights
need to be higher than that of a RED signal light.
This is likely due to the Helmholtz-Kohlrausch
effect.
Federal Highway Administration
Department of Transportation
Review of prior research
Effects of color on intensity requirements
• The Helmholtz-Kohlrausch effect is when a
chromatic stimulus appears to have a greater
brightness than a white reference stimulus of the
same luminance. The ratio of the luminance of the
white reference stimulus to the luminance of the
chromatic stimulus, with equal brightness is noted
as B/L. The effect varies by wavelength and
saturation, with highly saturated reds and blues
having a higher B/L value than yellows or greens.
Federal Highway Administration
Department of Transportation
Review of prior research
Effects of color on intensity requirements
• Adrian (1963), Rutley, Christie & Fisher (1965),
Jainski & Schmidt-Clausen (1967) and Fisher
(1971) found that, for both daytime and nighttime
viewing, the ratio of the luminous intensity of a
GREEN to RED signal light should be 1 : 1.3.
• Fisher & Cole (1974) evaluated several studies
and found that there was no conclusive results for
the ratio of YELLOW to RED, but indicated that
the evidence suggested a ratio of 1 : 3 was needed.
Federal Highway Administration
Department of Transportation
Traffic Light Intensity Requirements
200mm signal—daytime
• The “Fisher Equation” provides the luminous intensity
requirements for a 200mm (8 in.) RED traffic signal light,
viewed under standard conditions during daytime. The
equation is modified using the various color ratios to
calculate the luminous intensity requirements for GREEN
and YELLOW traffic signal lights.
• The various investigations found that, if observers where
not required to use a tracking task, and looked directly at
the signal light, the required luminous intensity values
were significantly lower.
Federal Highway Administration
Department of Transportation
Review of Prior Research
Validation
• NCHRP Project 5-15, “Visibility Performance
Requirements for Vehicular Traffic Signals,” was
intended to reevaluate prior research, and establish
performance requirements for traffic signal lights.
• The results of that effort found agreement with the
R:G ratio of 1 : 1.3, suggested a R:Y ratio of 1 : 2,
but found a higher basic requirement for luminous
intensity for the conditions tested.
Federal Highway Administration
Department of Transportation
Review of Prior Research
Validation
• The conditions evaluated under NCHRP 5-15 can be
considered a “difficult” driving task. The observer was
required to almost constantly monitor the tracking task,
and had minimal time for scanning the visual field. Thus,
the light signal had to have sufficient intensity to interrupt
their concentration on the driving task.
• The results of the project were that, for a difficult driving
task, the luminous intensity values need to be increased by
a factor of approximately 2.5 times.
Federal Highway Administration
Department of Transportation
Review of Prior Research
Validation
• The area of the emitting surface of a 300mm traffic signal
light is 2.25 times greater than that of a 200mm traffic
signal light. This is almost identical to the factor of 2.5
found in NCHRP Project 5-15.
• It is estimated that 70% of traffic signal lights in the U.S.
use 300mm signal lights. This indicates that local traffic
engineers have responded to public concerns for higher
intensity signals as roadway complexity has increased.
• The basic results of the research compiled within CIE
Publication No. 48 appear applicable to today’s
requirements, with the understanding that local traffic
engineers are best able to determine if a 200mm or
300mm traffic signal light is required.
Federal Highway Administration
Department of Transportation
Evaluation of Existing Standards
• The purpose of a traffic signal light has been
defined.
• The results of prior research have been evaluated,
and are found applicable to today’s driving task.
• The remaining task is to evaluate how well the
requirements in the ITE VTCSH meet the needs
of the driver to determine the requirement for
revising the standard.
Federal Highway Administration
Department of Transportation
Institute of Transportation Engineers
“Vehicle Traffic Control Signal Heads —Part 2”
(1998)
Photometric requirements for U.S. LED traffic lights
Luminous intensity & chromaticity
Peak Values of Luminous Intensity
(2.5° down & 2.5° to left and right)
200mm LED Signal
Red Yellow Green
133
617
267
300mm LED Signal
Red Yellow Green
339
1571
678
Ratio of R:Y:G—1:4.6:2
Based on historic (incandescent) signaling equipment!
(Requirements are minimum values throughout service life)
Federal Highway Administration
Department of Transportation
Evaluation of Existing Standard
Comparison to Fisher equation
• An initial look at the peak luminous intensity
requirements, established at an angle of 2.5 deg
below the horizontal plane and 2.5 deg to the left
and right of the normal of the signal face, indicate
that the maintained values for RED are low, and
the ratios for R:G and R:Y are too high.
• The second evaluation was to compare the beam
distribution with results using the Fisher
equation.
Federal Highway Administration
Department of Transportation
Intensity Coefficient vs Eccentricity Angle
(horizontal and vertical offsets of 3.2 meters)
Fisher Equation
Intensity Coefficient versus Fisher
Eccentricity Angle
25
Intensity Coefficient
20
15
ITE
10
5
0
0
5
10
15
20
25
30
Eccentricity Angle (degrees)
Federal Highway Administration
Department of Transportation
35
Evaluation of Existing Standard
Comparison to Fisher equation
• The preceding graph show fairly good agreement
with the requirements of the ITE VTCSH and the
Fisher equation up to approximately 16 degrees
of eccentricity. After that, the ITE requirements
drop off rapidly, as compared to those calculated
with the Fisher equation.
• The data were reevaluated, to compare the results
by the distance between the signal light and the
driver.
Federal Highway Administration
Department of Transportation
Intensity Coefficient vs Line-of-Sight Distance
25
Intensity Coefficient
20
Fisher Equation
Intensity Coefficient versus Line-of-Sight Distance
Fisher
15
ITE
10
5
0
0
10
20
30
40
50
60
70
Line-of-Sight Distance (meters)
Federal Highway Administration
Department of Transportation
80
Evaluation of Existing Standard
Comparison to Fisher equation
• The second graph shows that the requirements of the ITE
VTCSH follow the Fisher equation, until the line-of-sight
distance is less than 20 m. Since this distance is less than
the stopping distance at almost any reasonable speed,
there is no need to notify the driver of a change in signal
at the corresponding eccentricity angles.
• In fact, if the requirements of the ITE VTCSH continued
to follow the Fisher equation, the intensity of a signal
light would be unbearably high when a vehicle is stopped
at the stop bar of an intersection.
Federal Highway Administration
Department of Transportation
Evaluation of Existing Standard
Comparison to Fisher equation
• The comparison of the requirements of the ITE
VTCSH and the Fisher equation indicate a need to
balance viewing angle requirements with realworld geometry.
• The ITE VTCSH substantially follows the Fisher
equation for typical viewing angles, and
compensates for potential glare. Furthermore, the
ITE VTCSH has the benefit of time-in-service
with no indications of major deficiencies.
Federal Highway Administration
Department of Transportation
Evaluation of Existing Standard
Complete beam profile
• The beam profile requirements of the ITE VTCSH
are, however, limited to beam angles from 2.5 deg
below the horizontal plane and below. This is a
legacy of the incandescent system. Incandescent
signal lights could not reasonably be configured to
emit no light above the 2.5 deg downward angle,
while LED signal lights can. The possible
placement of signal lights on steep approaches and
use of wire-span mounted signal heads in windy
areas necessitated development of a more
complete beam profile.
Federal Highway Administration
Department of Transportation
Evaluation of Existing Standard
Complete beam profile
• There was no information available on the
variance in intended orientation of a signal head
and the actual orientation under windy conditions.
• As a result, measurements of the full profile of a
compliant incandescent traffic signal lights were
made as a means of establishing an existing
performance level.
Federal Highway Administration
Department of Transportation
Evaluation of Existing Standard
Complete beam profile
• The following graphs illustrate the normalized,
averaged horizontal beam profiles for several
incandescent traffic signal lights, at various
vertical angles.
– The first graph shows all of the measured profiles.
– The second graph averages the average profiles, based
on general curve shape.
– The third graph presents a final horizontal beam profile,
based on the most critical vertical angles, and a fit to
that profile.
Federal Highway Administration
Department of Transportation
Normalized Horizontal Profiles
1.20
Existing Beam Profile
Normalized Horizontal Profiles
Normalized Luminous Intensit
1.00
0.80
0.60
22.5 deg UP
17.5 deg UP
12.5 deg UP
7.5 deg UP
2.5 deg UP
0.40
2.5 deg DWN
7.5 deg DWN
12.5 deg DWN
17.5 deg DWN
22.5 deg DWN
0.20
27.5 deg DWN
32.5 deg DWN
37.5 deg DWN
0.00
-30
-20
-10
0
10
20
30
Horizontal Angle (degrees)
Federal Highway Administration
Department of Transportation
Group-Averaged Normalized Horizontal Profiles
1.20
Normalized Luminous Intensit
1.00
Existing Beam Profile
Group-Averaged Normalized Horizontal Profiles
0.80
0.60
22.5 to 7.5 UP
2.5 UP to 7.5 DWN
0.40
12.5 to 27.5 DWN
32.5 & 37.5 DWN
0.20
0.00
-30
-20
-10
0
10
20
30
Horizontal Angle (degrees)
Federal Highway Administration
Department of Transportation
Normalized Horizontal Profile & Fit
1.20
Averaged Normalized Horizontal
Profile & Fit
f (I ) =0.05+0.9434*e
(-0.5*(θhoriz/ 11)2)
horiz
Profile
Fit
Normalized Luminous Intensity
1.00
0.80
0.60
0.40
0.20
0.00
-40
-30
-20
-10
0
10
20
30
40
Horizontal Angle (degrees)
Federal Highway Administration
Department of Transportation
Evaluation of Existing Standard
Complete beam profile
• The next three graphs illustrate the normalized,
averaged vertical beam profiles for several
incandescent traffic signal lights, at various
horizontal angles.
– The first graph shows all of the measured profiles.
– The second graph is of a weighted average of the
profiles, with horizontal angles closer to 0 given greater
weight.
– The third graph presents the final vertical beam profile,
and a fit to that profile. Note that the fit required two
separate equations, with a break at 2.5 deg down.
Federal Highway Administration
Department of Transportation
Normalized Vertical Profiles
1.20
Existing Beam Profile
Normalized Vertical Profiles
2.5 deg
1.00
7.5 deg
12.5 deg
17.5 deg
Normalized Luminous Intensit
22.5 deg
27.5 deg
0.80
0.60
0.40
0.20
0.00
-40
-30
-20
-10
0
10
20
Vertical Angle (degrees)
Federal Highway Administration
Department of Transportation
Averaged Vertical Profile
1.20
Existing Beam Profile
Averaged Vertical Profiles
1.00
Normalized Luminous Intensit
0.97
0.82
0.80
0.60
0.40
0.32
0.23
0.20
0.16
0.10
0.06
0.06
0.12
0.12
0.10
0.07
0.08
0.00
-40
-35
-30
-25
-20
-15
-10
-5
0
5
10
15
20
25
Vertical Angle (degrees)
Federal Highway Administration
Department of Transportation
Normalized Vertical Profile & Fit
Averaged Vertical Profile & Fit
1.20
θvert > -2.5 deg: 0.05+0.9434*e(-(θvert +2.5)/5.3)
2 (-0.07* θvert)
θvert <= -2.5 deg: 0.3+(0.007*θvert )+0.72*[e(-0.02*(θvert +2.5) ]
Normalized Luminous Intensity
1.00
Profile
Fit
0.80
0.60
0.40
0.20
0.00
-40
-35
-30
-25
-20
-15
-10
-5
0
5
10
15
20
25
Vertical Angle (degrees)
Federal Highway Administration
Department of Transportation
Minimum maintained intensity
requirements
I(θHoriz, θVert, size, color) = f(IHoriz)* f(IVert)*I(-2.5, 0).
All values of θHoriz
2⎞
⎛ 1 ⎛θ
⎛
⎜ − *⎜ Horiz ⎞⎟ ⎟ ⎞
f (I Horiz ) = 0.05 + ⎜ 0.95∗e⎜⎝ 2 ⎝ 11 ⎠ ⎟⎠ ⎟
⎟
⎜
⎠
⎝
θVert > -2.5 deg.
f (IVert ) = 0.05 + 0.9434∗e
θVert ≤ -2.5 deg.
⎛
−⎜
⎜
⎝
θ
+ 2.5 ⎞
⎟
⎟
5.3
⎠
Vert
f (IVert ) = 0.26 + ⎛⎜ θ Vert ⎞⎟ + 0.76* ⎡⎢e −0.02(θVert + 2.5 ) ⎤⎥
⎝ 143 ⎠
⎣
⎦
2
(−0.07*θVert )
Federal Highway Administration
Department of Transportation
Minimum maintained intensity
requirements
I(θHoriz, θVert, size, color) = f(IHoriz)* f(IVert)*I(-2.5, 0).
I(-2.5,0) =
Color
Red
Yellow
Green
200mm
165 cd
410 cd
215 cd
300mm
365 cd
910 cd
475 cd
Federal Highway Administration
Department of Transportation
Evaluation of Existing Standard
Color requirements for signal lights
• The ITE VTCSH color requirements were
compared to the requirements of CIE Standard
S004 (2001), “Colours of Light Signals”.
Federal Highway Administration
Department of Transportation
Evaluation of Existing Standard
Color requirements for signal lights
• ITE VTCSH color region for red does not have a long
wavelength cutoff, permitting very deep reds that are not
readily detectable by some CVD driver…CIE S 004
provides a recommended cut-off.
• ITE VTCSH color regions for red and yellow have very
little room for manufacturing tolerances…CIE S 004
provides greater tolerance without loss of color
recognition.
• ITE VTCSH and CIE S 004 color regions for green allow
unsaturated colors that can blend into the sky, and include
longer wavelength greens that may be of undetermined
color by some CVD drivers.
Federal Highway Administration
Department of Transportation
Evaluation of Existing Standard
Color requirements for signal lights
• A modified version of the 1931 CIE Chromaticity Diagram
illustrates the concern with long wavelength green signal
lights. This figure shows the limited color discrimination
for dichromates…those people who lack either the middle
(green) or long (red) wavelength cone detectors in the eye.
• This affects approximately 2% of males of European
descent. An additional 6% of males have reduced
sensitivity for green and red stimuli, which may result in
color confusion or missing a signal. Less than 1% of
females have a color vision deficiency.
Federal Highway Administration
Department of Transportation
Evaluation of Existing Standard
Color requirements for signal lights
Ye
ll
Pale
Yellows
ow
Red Signal
seen as
yellow
Green signal
seen as
bluish-white
White
Pale
Blues
Blue signal
seen as
blue
Federal Highway Administration
Department of Transportation
Evaluation of Existing Standard
Color requirements for signal lights
• By moving the long wavelength cut off of the green signal
light closer to the “white” line for dichromate observers,
the green signal will be more readily distinguished from
yellow and red.
• A second concern was to establish a long wavelength cut
off for red signal lights as well. With an incandescent
system, no one would use a filter that produces a signal
light with a dominant wavelength above 630 nm, as it
would be very inefficient. With direct emitting sources,
such as LEDs, however, it is relatively easy to generate a
very deep red signal light.
Federal Highway Administration
Department of Transportation
Evaluation of Existing Standard
Color requirements for signal lights
• The following graphs show the existing color regions for
traffic signal lights in the ITE VTCSH, and proposed color
regions for an LED supplement to the ITE VTCSH. In
actuality, it is desirable that all traffic signal lights use the
newly developed color regions, to better meet the needs of
all drivers.
Federal Highway Administration
Department of Transportation
Color Areas for Green Traffic Lights
(1931 CIE Chromticity Diagram)
0.90
Chromaticity Diagram
0.80
ITE VTCSH
510
LED Lights
0.70
Incandescent
ITE VTCSH LED
0.60
500
y
0.50
0.40
0.30
490
0.20
0.10
0.00
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
x
Federal Highway Administration
Department of Transportation
0.35
Color Areas for Red Traffic Lights
(1931 CIE Chromticity Diagram)
Chromaticity Diagram
ITE VTCSH
610
LED Lights
0.33
Incandescent
ITE VTCSH LED
0.31
y
620
630
0.29
640
0.27
0.25
0.64
0.66
0.68
0.70
0.72
0.74
x
Federal Highway Administration
Department of Transportation
0.50
Color Areas for Yellow Traffic Lights
(1931 CIE Chromticity Diagram)
580
Chromaticity Diagram
0.48
ITE VTCSH
LED Lights
Incandescent
ITE VTCSH LED
0.46
y
0.44
590
0.42
0.40
0.38
0.36
0.50
0.52
0.54
0.56
0.58
0.60
0.62
0.64
x
Federal Highway Administration
Department of Transportation
Evaluation of Existing Standard
Luminance uniformity
• The final consideration in evaluating the existing ITE
VTCSH was in defining the uniformity of appearance of a
traffic signal light. The existing ITE VTCSH does not have
such a requirement. There was concern that the pixilated
appearance of some LED signal lights might reduce the
effectiveness of the signal.
• Greenhouse & Cohn (1999) compared the response to
various LED signals and a uniform incandescent signal,
and found that for equivalent intensity, the LED signals
performed as well or better than the incandescent signal.
• NCHRP Project 5-15, however, found that drivers did not
like signal lights with max:min ratios greater than 10:1.
Federal Highway Administration
Department of Transportation