Turn-On and Turn-Off Characteristics
of Incandescent and Light-Emitting
Diode Signal Modules
REPLACING INCANDESCENT
INTRODUCTION
APPROACH MAY CREATE
Traffic signal light sources either are
incandescent or light-emitting diode
(LED). LED-based traffic signal modules
have a higher capital cost but offer
tremendous energy savings because they
consume less power and do not require
annual preventive replacement. Consequently, LED modules are rapidly replacing incandescent modules.
However, using LED modules from
different manufacturers on a given
intersection approach may create confusion for drivers because the same color
indicators on different signal heads may
not start (or end) simultaneously. It was
reported that, at an intersection
approach with LED modules from different manufacturers, motorists noticed
that one of the green modules came on
before the other.
The authors conducted this study to
investigate the turn-on and turn-off characteristics of incandescent and LED traffic signal modules.
CONFUSION FOR
DATA COLLECTION AND REDUCTION
TRAFFIC SIGNAL LIGHTS AT
INTERSECTIONS WITH
LIGHT-EMITTING DIODE
(LED) MODULES IS
BECOMING MORE
COMMON IN THE UNITED
STATES. HOWEVER, USING
LED MODULES FROM
DIFFERENT
MANUFACTURERS ON THE
SAME INTERSECTION
Incandescent lights (DURA-TEST
TRAVELERS AND
lamps) and LED modules manufactured
by Gelcore and Dialight were used in this
ADVERSELY AFFECT
investigation. LED modules from these
two manufacturers were evaluated
TRAFFIC SAFETY.
because they are the main suppliers of
LED traffic signal modules for the Illinois Department of Transportation.
At the Traffic
BY MADHAV V. CHITTURI, RAHIM F. BENEKOHAL, PH.D. Operations Laboratory at the University
AND MONTTY GIRIANNA, PH.D.
of Illinois, a frame
was set up with three signal heads next
to each other (one incandescent and two
LEDs). They were programmed to turn
on and turn off simultaneously using a
National Electrical Manufacturers Association (NEMA) controller.
ITE JOURNAL ON THE WEB / OCTOBER 2005
Data were collected for 114 cycles of
92 seconds in duration. The operation
was videotaped to achieve one-thirtiethsecond time accuracy. More detailed
information on data collection and
experimental setup can be found in
Benekohal, Girianna and Chitturi.1
The videotapes were time-coded and
played at a slow speed so that each frame
(one-thirtieth-second) could be viewed
individually.
Traffic signal modules do not completely illuminate or darken instantaneously. For each module, the observer
recorded the following times:
• Time the module started to turn on;
• Time the module was fully on;
• Time the module started to turn off;
and
• Time the module was fully off.
The terms used in this study are defined
in the following section. A 95-percent
confidence level was used for all statistical tests.
DEFINITIONS
Fade-in time is the time required for
a module to reach full brightness. In
Figure 1, fade-in time is represented by
the hashed area between the points
labeled “start on” and “fully on.” Fadeout time is the time required for a module to reach complete darkness.
Fade-out time is represented by the
hashed area between the points labeled
“start off ” and “fully off.”
Turn-on (or turn-off) delay quantifies
whether the LED modules start turning
on (or turning off) earlier or later than
the incandescent module. Turn-on delay,
dn, is the difference between “start on”
times; turn-off delay, df, is the difference
between “start off” times for the LED
modules and the incandescent module.
69
FINDINGS
Fade-In and Fade-Out Time
The ITE LED Vehicle Traffic Control
Signal Modules Draft Specification
defines turn-on time as the time delay
between application of nominal operating voltage and the module reaching 90
percent of full light output.2 It defines
turn-off time as the amount of time
required after removal of the nominal
operating voltage for the LED signal
module to show no visible illumination.
The ITE Draft Specification requires that
turn-on and turn-off times for LED signal modules not exceed 75 milliseconds.
Fade-in (and fade-out) times,
reported in milliseconds, for the three
types of traffic signal modules are summarized in Table 1. T-tests with 95-percent confidence were performed to
determine if fade-in (fade-out) times
were statistically different from zero. The
test results indicated that fade-in (fadeout) times for all three modules were
greater than zero.
Although the ITE turn-on (turn-off)
times were similar to the fade-in (fadeout) times defined in this study, there
were certain minor differences. The ITE
turn-on time requires the signal module
to reach 90 percent of full illumination.
In this study, fade-in time requires the
module to reach full brightness as perceived by an observer. The ITE Draft
Specification procedure uses a two-channel oscilloscope for measuring time
delays. In this study, the times were measured by an observer viewing the video at
a slow speed. Therefore, fade-in time
would be expected to be higher than
turn-on time for any module.
The evaluated LED modules met the
fade-in time
fade-out time
Fully on
Start on
Fully off
Start off
Incandescent
Start on
Start off
Fully on
Fully off
LED I
dn
Start on
df
Start off
Fully on
Fully off
LED II
df
dn
dn = Turn-on delay;
df = Turn-off delay
fade-in time
fade-out time
fully dark
fully illuminated
Figure 1. Definitions and terms used to determine turn-on and turn-off characteristics.
ITE Draft Specification requirements for
turn-on time. Statistical tests indicated that
the average fade-in times for all LED modules were less than 75 milliseconds. In fact,
the maximum fade-in time for all LED
modules, except green Gelcore, was less
than 75 milliseconds. The average fade-in
time for the red incandescent module was
statistically less than 75 milliseconds; however, the average fade-in times for the yellow and green incandescent modules were
greater than 75 milliseconds.
The average fade-out times for all the
modules were statistically greater than 75
milliseconds. These high values could be
due to the fact that human observers made
the measurements and that the fade-out
time defined in this study was measured
from the time the modules start dimming
to the time they were completely dark.
Comparing fade-in (fade-out) times
of incandescent lamps to the LED modules indicated that, for any given color,
the fade-in time for incandescent was
greater than that for the Gelcore LED,
and the fade-in time for the Gelcore LED
was greater than that for the Dialight
LED. The differences were statistically
significant (see Table 2).
Similar comparisons for fade-out
times indicated that the red incandescent
had the longest fade-out time, followed
by the Gelcore LED and Dialight LED.
For green indications, the Dialight LED
had the longest fade-out time, followed
by the incandescent and the Gelcore
LED. These differences were statistically
significant. For yellow, the fade-out time
for incandescent was the longest and statistically different from the LED modules; the fade-out times for the two yellow
LED modules were statistically the same.
Table 1. Fade-in and fade-out times for incandescent and LED modules.
Incandescent
Color
Fade-in time
(in milliseconds)
Fade-out time
(in milliseconds)
Gelcore LED
Minimum Average Maximum t-value*
Red
Yellow
Green
Red
Yellow
Green
33
133
100
500
433
333
71
169
143
602
501
377
100
200
167
767
600
467
36.88
83.55
89.72
120.68
139.98
164.94
Dialight LED
Minimum Average Maximum t-value* Minimum Average Maximum t-value*
33
33
33
367
167
167
40
39
63
403
222
227
67
67
100
467
300
267
32.14
33.28
38.72
137.34
92.21
96.45
33
33
0
133
200
433
34
34
36
208
220
475
67
67
67
267
267
533
82.39
115
35.91
53.88
104.73
247.56
* Note: Critical ‘t’-value = 1.6584.
70
ITE JOURNAL ON THE WEB / OCTOBER 2005
Turn-On and Turn-Off Delay
Turn-on delay quantifies whether LED
modules start to turn on earlier or later
than the incandescent module. Similarly,
turn-off delay quantifies whether LED
modules start to turn off earlier or later
than the incandescent module. The times
at which the incandescent module starts to
glow (start on) and starts to dim (start off)
are the benchmarks for measuring turn-on
and turn-off delays, respectively. Positive
values for turn-on (turn-off) delays mean
that the LED modules start to turn on
(turn off ) later than the incandescent
module. Negative values mean that they
turn on (turn off) earlier.
In Table 3, the turn-on (turn-off )
delays for the LED modules are documented. All turn-on delays were statistically different from zero. With the
exception of the red Dialight, the turn-on
delay for all LED modules was zero or
positive. In other words, the LED modules started to turn on some time after the
incandescent module. For the red Dialight LED, the delay was negative.
On average, the turn-off delay for all
LED modules other than the green Gelcore
was negative. In other words, LED modules
other than the green Gelcore start turning
off earlier than the incandescent module.
For any color, the Dialight LED module had a smaller turn-on (turn-off) delay
than the Gelcore LED module. When the
video was played at a regular pace, it was
observed that for the green color, the Dialight module came on earlier than the Gelcore module. This supported the problem
drivers had reported about the intersection
approach with different LED modules.
However, there was no such perceptible difference for the yellow and red LED
Table 2. Summary of statistical tests for fade-in and fade-out times.
Fade-in time
Fade-out time
Color
Statistical results
Red
Yellow
Green
Red
Yellow
Green
Incandescent > Gelcore LED > Dialight LED
Incandescent > Gelcore LED > Dialight LED
Incandescent > Gelcore LED > Dialight LED
Incandescent > Gelcore LED > Dialight LED
Incandescent > Gelcore LED = Dialight LED
Dialight LED > Incandescent > Gelcore LED
modules. The practical implication of
this finding is that one should not mix
green LEDs from Gelcore and Dialight
on the same intersection approach, but
mixing yellow or red LED modules from
the two manufacturers would not create a
noticeable delay.
Overlaps and Gaps
If modules have perceptibly different
turn-on delays when the signal changes
from one phase to another, motorists
might find an intersection with no signal
glowing or signals of two different colors
glowing at the same time. Phase transitions from green to yellow and yellow to
red were studied. These two signal
changes are critical from a safety point of
view. Two situations that could create a
potential traffic safety hazard are:
• No signal indications are on at the
intersection for a very brief time (the
intersection is momentarily dark).
This situation would happen if the
modules for the first phase are fully
off and the modules for second phase
have not yet started to turn on. This
instance is represented by a gap in
case 1 (fully off and start on are the
reference points for phases 1 and 2,
respectively), as shown in Figure 2.
• Two different color signal indications are on simultaneously for the
same approach at the intersection
(dual indication). This situation
arises when the modules for the first
phase have not turned off completely
and the modules for the second
phase are fully on. This situation is
represented by an overlap in case 2
(start off and fully on are the reference points for phases 1 and 2,
respectively), as shown in Figure 2.
The time difference between the reference points for phases 1 and 2 is computed as . The overlaps/gaps, ,
between the signal phase transitions green
to yellow and yellow to red are presented
in Table 4. The first two columns show the
nine possible combinations of signal modules. Gaps and overlaps result in negative
and positive values for , respectively. All
the overlaps of case 1 and all the gaps of
case 2 are statistically different from zero.
Potentially unsafe traffic conditions
could be created when gaps occur in case 1
or when overlaps occur in case 2. In both
the phase transitions studied, no gaps were
observed in case 1 and no overlaps were
observed in case 2. Therefore, for any
combination of the modules investigated,
Table 3. Turn-on and turn-off delays (in milliseconds) for LED modules.
Gelcore LED
Color
Turn-on delay
Turn-off delay
Dialight LED
Minimum Average Maximum t-value*
Red
Yellow
Green
Red
Yellow
Green
0
33
100
-133
-100
-33
46
69
149
-16
-3
17
100
133
200
33
0
67
27.46
58.46
86.66
-5.46
-2.41
9.23
Difference
Minimum Average Maximum t-value*
-33
0
33
-133
-100
-67
-21
36
36
-32
-7
-6
0
67
67
33
33
33
-13.66
34.08
44.54
-9.1
-3.86
-3.59
in average#
t-value*
67
33
113
16
4
23
29.56
20.74
59.83
6.81
2.75
12
* Note: Critical ‘t’-value = 1.6584; # critical ‘t’-value = 2.414.
ITE JOURNAL ON THE WEB / OCTOBER 2005
71
Figure 2. Overlaps and gaps for cases 1 and 2.
two signals were not glowing simultaneously and there was no dark intersection.
Some readers may be concerned about
the overlaps observed in case 1 and the
gaps observed in case 2. To address these
concerns, the investigators viewed the
videotapes at regular speed and observed
that in none of these cases could the viewers see multiple signals glowing simultaneously or none of the signals illuminated.
CONCLUSIONS
This feature examined the fade-in and
fade-out times, turn-on and turn-off
delays and overlaps/gaps of colors
between consecutive phase intervals for
incandescent lamps and LED modules.
For all colors, the LED modules had
shorter fade-in times when compared
with the incandescent module. The fadein time for the Dialight LED was shorter
than for the Gelcore LED, for all the
three colors. The fade-in times of all the
LED modules evaluated in this study
were less than the ITE Specification
requirement for turn-on time.
The Dialight LED module had the
longest fade-out time for green. The
incandescent module had the longest and
the Dialight LED modules had the shortest fade-out times for yellow and red. The
fade-out times for all the evaluated mod-
ules were greater than the ITE Specification requirement for turn-off time. This
difference could be due to the difference
in the measurement approach.
Except in the case of the red Dialight,
all LED modules started to turn on a
fraction of a second later than the incandescent modules. All of the LED modules other than the green Gelcore started
to turn off a fraction of a second earlier
than the incandescent modules. In the
lab, an observer could note that the green
Dialight module came on before the
green Gelcore module. However, for yellow and red, the difference in LED modules was not noticeable.
Two situations could create unsafe
traffic conditions: when approaching
motorists see two different color signal
indications turned on simultaneously,
and when no signal indication is on at
the intersection. Statistical tests and
visual observations confirmed that these
situations did not happen. That is, none
of the combinations of the traffic signal
modules studied could result in multiple
signals indications being displayed or
none of the signals being illuminated. In
Table 4. Summary of gaps/overlaps (in milliseconds) between consecutive phases.
Signal module combination
Phase change
Green
Yellow
Green to yellow
Incandescent
Incandescent
Incandescent
Gelcore LED
Gelcore LED
Gelcore LED
Dialight LED
Dialight LED
Dialight LED
Incandescent
Gelcore LED
Dialight LED
Incandescent
Gelcore LED
Dialight LED
Incandescent
Gelcore LED
Dialight LED
Yellow
Red
Incandescent
Incandescent
Incandescent
Gelcore LED
Gelcore LED
Gelcore LED
Dialight LED
Dialight LED
Dialight LED
Incandescent
Gelcore LED
Dialight LED
Incandescent
Gelcore LED
Dialight LED
Incandescent
Gelcore LED
Dialight LED
Yellow to red
Case 1: Overlaps/gaps from fully off to start on
Case 2: Overlaps/gaps from start off to fully on
Minimum
Average
Maximum
t-value*
Minimum
Average
Maximum
t-value*
300
233
267
167
100
133
400
333
367
358
289
322
226
156
189
450
381
414
433
367
400
300
233
267
500
433
467
117.56
92.4
104.68
102.65
67.51
85.73
211.47
173.55
185.59
-233
-167
-133
-233
-167
-133
-233
-167
-133
-188
-127
-89
-171
-110
-72
-194
-133
-94
-100
-67
-33
-100
-67
0
-100
-100
-33
-72.37
-61.79
-45.98
-57.49
-51.98
-38.35
-72.89
-88.71
-68.82
400
333
400
167
100
167
167
100
167
483
437
504
201
154
222
194
148
215
633
567
633
300
233
300
233
200
267
106.95
95.96
113.73
73.36
54.65
81.39
95.73
72.66
112.69
-133
-167
-100
-133
-167
-100
-133
-167
-67
-89
-105
-32
-92
-108
-35
-97
-112
-39
0
0
67
-33
-67
0
-33
-67
-33
-30.99
-42.08
-16.27
-37.06
-51.78
-25.13
-43.29
-58.66
-33.28
* Note: Critical ‘t’-value = 1.6584.
72
ITE JOURNAL ON THE WEB / OCTOBER 2005
other words, the unsynchronized turn-on
and turn-off characteristics did not result
in these two situations that could compromise traffic safety. ■
References
1. Benekohal, R., M. Girianna and M. Chitturi. “A Comparison Between the Turn-On and
Turn-Off Characteristics of Incandescent and
LED Traffic Signal Modules.” University of Illinois Civil Engineering Studies, Transportation
Engineering Series, No. 121, December 2003.
2. Purchase Specification of the Institute of
Transportation Engineers. “Vehicle Traffic Control Signal Heads—Part 2: Light Emitting Diode
Vehicle Traffic Signal Modules.” December 2003.
MADHAV V.
CHITTURI
is a doctoral candidate in
the Department of Civil
and Environmental
Engineering at the
University of Illinois at
Urbana–Champaign.
He graduated from the Indian Institute of Technology
in Kharagpur, India, with a bachelor’s degree and the
University of Illinois at Urbana–Champaign with a
master’s degree in civil engineering. His research interests include traffic simulation, traffic modeling and
traffic safety.
ITE JOURNAL ON THE WEB / OCTOBER 2005
RAHIM F.
BENEKOHAL,
Ph.D., is a professor of
civil engineering at the
University of Illinois at
Urbana–Champaign.
Previously, he worked for
the consulting firm of
RKA Inc. He received a B.S., M.S. and Ph.D. from
the Ohio State University. He teaches and conducts
research in traffic modeling, ITS evaluation, transportation safety and transportation systems analysis.
He is the winner of the Arthur Wellington Award
and ITE Illinois Section Past President Award. He is
director of the Traffic Operations Lab at the university as well as director of the Illinois Traffic Engineering and Safety Conference. He is a member of ITE.
MONTTY
GIRIANNA,
Ph.D., graduated from
Bandung Institute of
Technology in 1985
with a degree in civil
engineering. For several
years, he worked for the
National Development Planning Agency (BAPPENAS) of the Republic of Indonesia before pursuing
graduate studies in urban planning at Massachusetts Institute of Technology and a Ph.D. in transportation systems at the University of Illinois at
Urbana–Champaign. He is now working for
BAPPENAS as chief of infrastructure financing.
73