Evaluation
of Traffic Signal Displays for
Protected-Permit
ted Left-Turn Control
NCHRP Project 3-54
NOTICE: This report is being furnished to the traffic engineering community at the direction
of NCHRP so to supply information about this research project and its findings. This report
documents findings of the research project as of September 1999.
Date submitted to the Project Web page: February 2000
PHOTOGRAPHIC DRIVER STUDY REPORT
Working Paper 3
Prepared for:
National Cooperative Highway Research Program
Project Panel Members
August 1999
Prepared by:
KITTELSON & ASSOCIATES, INC.
TEXAS TRANSPORTATION INSTITUTE
2200 W. Commercial Boulevard, Suite 304
Texas A&M University System
Ft. Lauderdale, FL 33309
College Station, TX 77843-3135
(954) 735-1245, FAX (954) 735-9025
(409) 845-1717, FAX (409) 845-6481
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Working Paper 3
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Page 1
PHOTOGRAPHIC DRIVER STUDY REPORT
INTRODUCTION
Transportation engineering encompasses a wide array of topics related to the movement of people and
goods. Given this broad diversity, very few components of transportation engineering, including the design
and implementation of traffic signal displays, can be fully developed without an understanding of human
factors. Incorporating human factors applies information about human behavior, abilities, limitations, and
other characteristics to the design of transportation environments for productive, safe, and effective human
use ( 1 ). In relation to traffic signal design, human factors can be defined as how drivers accomplish driving-
related tasks in the context of driver-vehicle system operation, and how behavioral variables affect this
accomplishment ( 2 ). Further, in controlling vehicle movements at signalized intersections, analyzing human
factors provides insight and understanding into how drivers process and react to traffic signal display
information.
The general human factors concepts that affect the design of traffic signal displays include visual search
processes, driver perception and reaction, driver recognition and comprehension, driver expectancy, and
signal complexity.
Visual Search
When a driver is looking for a traffic signal display indication (target) in an approaching intersection
environment, the visual scan pattern tends to be far less structured and organized than in other control tasks
( 3 ). The driver's task relative to traffic signal displays involves detection of the signal display, which is
inhibited by ambiguous signal meanings and conflicting and inconspicuous displays ( 4 ). Consequently, it
is difficult to model the driver's target search process.
Understanding a driver's target search process is compounded when age is considered because age
negatively influences memory scanning and visual search capabilities, particularly in terms of accessing
information needed for left-turn decision making ( 5 , 6 , 7 ). Nevertheless, target search can be described
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as driven in part by cognitive factors related to the expectancy of where in the visual field a target or the
most useful information is likely to be found.
In a 1972 study on the target search process, Mourant and Rockwell studied differences in target
search and scan patterns between novice and experienced drivers ( 8 ). Thirty drivers participated in this
study, which evaluated eye movement during the driving task. The researchers concluded that experienced
drivers had a wider visual scan pattern and obtained additional information from cues located further down
the roadway. The novice driver was found to act in quite the opposite manner with a very limited visual
scan pattern. One can conclude that the experienced driver's ability to attend to more sources of
information would allow for earlier recognition and response to traffic signal displays.
The fact that much of the visual search behavior is internally driven by cognitive factors means that there
are no highly consistent physical patterns of display scanning and no optimal scan pattern, beyond the fact
that search should be guided by the expectancy of the target location ( 3 ). There is little doubt that the
nature of traffic signal indications with large, bright colored indications draws visual attention, yet there may
be more subtle factors related to the physical locations of the display.
Megew and Richardson determined that subjects who conducted systematic scan patterns when
searching for visual targets tended to start in the upper left part of the display ( 9 ). Others have found that
search patterns may in fact begin in the center of the display, avoiding the outer edges ( 10 ). The problem
lies in the fact that search tendencies are neither consistent nor strongly dominated by internally driven scan
strategies ( 11 ). Further, driver response time and errors also increase with the number of action and
display choices within the associated search patterns ( 12 ).
Perception and Reaction
Understanding human issues related to drivers' perceptions and reactions is important in understanding
the effectiveness of a traffic signal display. Woodson has identified the following principles that enhance
perception and reaction to traffic signal displays ( 13 ).
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Conspicuity. The traffic signal display should attract attention and be located where drivers are
looking. The three major factors that determine the level of attention drivers devote to a traffic signal
display are prominence, novelty, and relevance.
Emphasis. The most important information-carrying aspects of the traffic signal display should be
emphasized or highlighted in some way. For example, the size or the intensity of the display can be
increased.
Visibility. The traffic signal display should be visible under all expected viewing conditions.
Maintainability. The traffic signal display structural design, material composition, and the associated
placement should be chosen to minimize damage as a result of aging, environmental wear, or vandalism.
Legibility and Intelligibility. The legibility, or contrast between the traffic signal display indications
and their background should be maximized. The traffic signal display should tell the driver what to do and
when to do it.
Standardization. Standard and consistent traffic signal displays and associated indications within each
display should be used.
Legibility and intelligibility affect drivers' processes of extracting necessary information to make
appropriate decisions. A key issue often associated with intelligibility is whether supplemental information
is necessary to assist the symbolic depiction of the signal display indication. Study results documented in
the literature are not consistent on this subject ( 7 , 14 , 15 , 16 , 17 ). Nevertheless, the signal display
configuration and operation that provides the greatest level of legibility and intelligibility will also result in
the fastest reaction time ( 18 ). If, for example, two different displays are quite similar and have insignificant
differences in error scores, with A being reliably responded to in less time than B , then the A display is
superior. The effects of driver age must be considered in comparing the reaction time to signal display,
as Allen found that drivers over the age of 70 required 1.5 seconds of additional time for signal display
recognition ( 19 ).
Achieving swift and accurate perception requires that the appropriate signal display indication be
illuminated, the driver pays attention in an appropriate way, and the appropriate speed-accuracy criteria
has been chosen. Ample time to perceive and respond to the traffic signal display is part of the speed-
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accuracy trade-off model. Errors have been shown to increase when drivers are asked to respond rapidly
( 20 ).
In addition to speed of response, the correctness of the action taken by the driver is a vital measure.
Experience indicates that in both simulations and actual driving maneuvers, smooth negotiation of an
intersection in response to command information provided by the traffic signal indication is indicative of
good signal display design. If a driver hesitates before identifying the correct maneuver, there is a strong
indication that the traffic signal display presentation may not be as effective as it could be.
Display elements that conflict with prior or common experience at PPLT intersections should be
avoided ( 5 , 21 ). Avoiding such conflicting information is even more important in light of findings that older
drivers are less able to inhibit such previously well-learned responses ( 22 ). Any technique that will enhance
the driver's memory of signal characteristics should reduce sensitivity decrements and preserve a higher
overall level of signal detection sensitivity. These techniques may include increased conspicuity, reduced
uncertainty, and proper training of the signal observers ( 18 ). Improving the effectiveness of a traffic signal
display may be as simple as applying the associated display arrangement and indications in a standard and
consistent way.
Driver Expectancy
The literature contains many studies that have been conducted to determine drivers' recognition and
comprehension related to various types of signal displays ( 14 , 15 , 23 , 24 ). Surprisingly, researchers often
overlook some basic principles of drivers' interpretation of signal displays, including the important concept
of driver expectancy. The position of a traffic signal display, intersection geometry, traffic characteristics,
or operational considerations may not be the sole sources of driver's misinterpretation of signal display
indications, but may be compounded by the drivers' interpretation of the display and the expectancies
associated with scanning for the correct information. Australian researchers believe that the level of driver
expectancy is maximized when traffic signal displays are placed in a uniform and consistent fashion at
signalized intersections ( 25 ). Signal display visual search requirements are minimized and driver
comprehension related to the signal display improved.
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A common form of expectancy theory incorporates a vigilance paradigm , which is a common
application of signal detection theory ( 3 ). Under the vigilance paradigm, if a driver is required to detect
common signals over a given period, the likelihood of the driver responding incorrectly increases each time
the driver makes an error ( 3, 18 ). The reasons for this extend into adjustments of individual response
criterion in response to varying expectancies of target events. To reduce the negative effects of this
paradigm, display placements that reduce driver sensitivity and increase expectancy should be implemented.
Variability in signal displays lends itself to problems with detection theory and driver expectancy ( 26 ).
First, driver expectancy can be violated when a horizontally mounted signal display and corresponding lens
arrangement are observed at location A , followed by a completely different mounting and arrangement at
location B . Second, not only must drivers visually observe the signal display, they must also understand
the meaning of the signal indication and make the appropriate driving decisions. Finally, drivers must know
where to look to find the information that they desire and must interpret and select the correct information.
The need for uniformity in traffic signal displays as it relates to driver expectancy has been well
documented in previous studies ( 24 , 27 ). Expectancies will be formed by experience and training and will
affect driver's readiness to respond to common situations in predictable and successful ways. Expectancy
will also affect how drivers perceive and handle information, and the time required to process and react to
this information. Reinforced expectancies, through uniform presentation of traffic signal information, help
drivers respond rapidly and correctly to the intended control message. Unusual, unique, uncommon, or
inconsistent situations that violate expectancies lead to longer response times, inappropriate responses, and,
ultimately, to increased driver error ( 28 ).
Signal Display Complexity
Complexity is also pertinent in understanding how much of the visual field a driver is able to evaluate.
Engel found the probability of detection decreases when conspicuity decreases; specifically, when objects
were more than five degrees off the line of sight ( 29 , 30 ). Similar research in Australia found the probability
of detecting an object superimposed on a driving scene decreased significantly when the object was moved
from 6 degrees to 12 degrees off the line of sight ( 31 ).
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Allen and Hill looked at a wide array of factors that may lead to traffic signal complexity ( 4 ). In
evaluating 134 drivers in Pennsylvania, Washington, California, Florida, and Virginia, Allen found that
supplemental signing leads to the greatest deterioration in subject performance in terms of significant
misunderstanding and long response times. Signal indications that prohibit movements, such as red arrows
and red and yellow balls, generally gave the best performance. Protected indications gave the poorest
performance while permissive indications gave intermediate results. Traffic signal display complexity was
increased by unclear and ambiguous signal indications and supplemental signing. Consequently, Allen
identified the following as ways to decrease signal display complexity ( 4 ):
!
Minimize supplemental signing. Complex signing tends to increase driver response time and
misunderstanding;
!
Minimize simultaneous viewing of conflicting displays between separate approaches and/or
intersections. The use of louvers and optically programmed signal displays are
recommended in most situations;
! Make sure each sign display has a clear, unique, and unambiguous meaning. Each part of
the protected and permitted movements at an intersection should have a separate indication.
Control of one movement should not have to be implied from another signal indication; and
!
Make sure that the display for each movement is apparent through placement and all signal
displays are conspicuous. Each signal display should be uniformly located within the driver's
line of sight for the desired movement.
Allen and Hill concluded that some of the intricacies of traffic signal displays may not be appreciated
by some drivers, which adds to the complexity problem. For this reason, displays should not rely on
implied meanings or indicate multiple alternatives that require the driver to analyze multiple display
indications to select an action. This point was emphasized in a study that included 300 driving educators
in Texas ( 32 ), who indicated that complex left-turn signal displays were the most difficult traffic control
device for their students to comprehend.
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Signal Display Colors
The earliest traffic signal displays used variations of red, yellow, and green indications, largely
determined by the available color medium. Over time, indication colors were modified by manufacturing
process, experience, and characteristics of the traffic signal ( 33 ). Ultimately, detailed specifications were
developed by ITE, the Commission Internationale De L'Eclairage (CIE— the International Commission
on Illumination), and the Society of Automotive Engineers (SAE) ( 34 ). CIE further developed a
chromaticity diagram that defined the color boundaries (in nanometers) for red, yellow, and green signal
indications ( 9 , 46 ).
Today, the color of a traffic signal display is most often developed by placing a filter material over a
white light source. The intensity and chromaticity of the light emitted from a traffic signal are dependent on
the choice of filter material and are therefore interdependent. Changes in technology have led to the gradual
replacement of filtered light sources with light emitting diodes (LED). Even with LED technology and the
development of rigid color specifications, there remains no optimum green, yellow, and red indication color
for traffic signal displays ( 33 , 35 ).
The Human Eye
Color vision research has confirmed that there are two types of photoreceptors in the human eye: the
rod, which is responsible for reception of low level light, and the cone, which is responsible for reception
of higher levels of light and color perception ( 36 ). The rods contain one type of photopigment, maximally
sensitive at a wavelength of about 505 nanometers (nm) ( 37 ). Short wavelength sensitive cones (S cones)
are sensitive to wavelengths of approximately 420nm, medium wavelength cones (M cones) to wavelengths
of approximately 530nm, and long wavelength cones (L cones) to wavelengths of approximately 560nm
( 13 , 38 ). In general, the eye is sensitive to a band of wavelengths from approximately 400nm to 700nm.
The three types of color receptors in the eye are linked into a system with two separate color
channels—one red and green and the other blue and yellow, both on an achromatic channel ( 38 ). The
yellow response is actually created by the interaction of red and green photopigments. Related aspects of
the visibility of a traffic signal display and the function of the eye have been described by Adrian ( 39 ).
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Sekuler identified the many genetic and acquired abnormalities that lead to some form of color deficient
or anomalous vision ( 38 ). Genetic causes are related to deficiencies in the inherited X-chromosome at birth
while acquired abnormalities are related to ocular disorders such as glaucoma and the effects of diabetes.
Another common cause is aging—drivers over 50 begin to experience a yellowing of the crystalline lens
of the eye as a result of pigment accumulation.
Estimates of the number of drivers with some form of color deficient or anomalous vision vary, but
recent estimates indicate that approximately 10 percent of all men (8 percent of Caucasian; 5 percent of
Asiatic; and 3 percent of African/Native Americans) and 1 percent of women have some form of deficiency
( 38 ). Hurvich reported that about 7 percent of the Caucasian population has deutan-type (red-green)
defects, about 2 percent has protan-type (red-green) defects, and about 0.001 percent has tritan-type
(yellow-blue) defects ( 40 ).
A panel of experts, reviewing the federal vision standard for commercial motor vehicle carriers, recently
changed the color vision requirements from "ability to distinguish red, green, and yellow/amber" to "a safe
and effective response to traffic signals and devices" ( 41 ). This change was based on the opinion that
drivers with color deficiencies presented little risk to the traveling public since traffic signalization has been
standardized and drivers have many other cues for the operation of a vehicle in a safe and effective
manner. Further, there is no evidence to suggest that drivers with red-green deficiencies have worse driving
records that those without color deficiencies.
No research has been identified that correlates poor color visual performance and driver safety ( 42 ).
This lack of research may suggest that drivers with color vision abnormalities somehow compensate in their
driving behavior to overcome their deficiencies, or may simply reflect the difficulty in quantifying this
relationship. Regardless, compensating for drivers with color vision deficiencies increases the importance
of uniformity and consistency in traffic signal displays and indications.
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Older Drivers
Along with changes in visual abilities, the human aging process also leads to changes in cognitive
capabilities. Figure 1 is a model of the cognitive components involved in driving ( 3 ). According to
Wickens, the driver first samples a number of information sources either simultaneously or sequentially.
The driver then develops a hypothesis which is a prediction or forecast of a future event. Hypothesizing
requires interplay between long-term and working memory. After processing, a choice of actions is
developed and a response action selected.
The outcome of the decision, whether successful or not, can influence future decisions. Feedback
provides this learning mechanism and opportunities to modify memory. Wickens' model shows the parallel
cognitive processes involved in processing roadway information, and identifies specific cognitive
components that may define the location of age-related differences in driving performance capabilities.
Studies of working memory show an age effect that favors younger over older drivers. This age effect
probably results from a decline in memory storage capacity, reduced processing efficiency, or impaired
coordination in these functions. As shown in Figure 1, working memory interacts with the decision and
response-selection cognitive functions, and is continually updated as new information replaces previous
inputs. This function is critical to the left-turn driving task as signal displays and roadway features must be
sampled and stored temporarily to direct instant-to-instant vehicle control and the planning of downstream
maneuvers. Thus, older drivers will be most at risk in situations that require rapid mental operations for
vehicle control, such as the left-turn maneuver, especially when they are required to perform such
operations and retain other information for future use.
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MEMORY
RESPONSES
Long-Term
Memory
Working
Memory
STIMULI
Feedback
Attention
Resources
Decision
and
Response
Selection
Perception
Short-Term
Sensory
Store
Response
Execution
Figure 1. Theoretical Model of Cognitive Components in a Driver's Processing of Traffic
Signal Display Information.
A number of related research efforts point to an overall decline in physical performance parameters in
older drivers including ( 43 , 44 ):
! A decline in information processing ability starting at 45 years of age;
!
A decline in dual task performance after age 60;
! Memory decrements requiring more time to retrieve information from primary memory may
place older persons at risk in situations requiring rapid manipulation of information and short-
term memory loss may effect appropriate responses;
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! Attention-related decrements, leading to deficits in visual scanning and other maneuvers that
require rapid reorientation of attention and may result in difficulties refocusing attention quickly
enough to respond to changing stimuli;
!
Divided attention deficits that may affect the speed at which information is processed and the
prioritization of incoming information, often requiring more decision time (decision time can also
be effected by visual clutter); and
! A need for additional time (estimated to be 2 seconds) to determine what to do when crossing
an intersection and to decide if it is safe to turn left.
The literature provides only general guidance when considering human variables in the design of PPLT
signal displays. There is little consensus as to how drivers visually scan for traffic signal display information
and the optimal location for display information. Further, the variability in drivers' perceptions, reactions,
recognition, and comprehension of traffic signal displays as a result of dynamic factors such as age,
cognitive state, ocular attributes, and signal complexity limits the ability of previous research to provide
definitive results.
The literature does highlight a number of important considerations in signal display selection. The safe
and efficient movement of left-turn vehicles can be enhanced by limiting the informational demands on the
driver. It has been shown that drivers can only comprehend a limited amount of information at a time, and
that information handling capacity is reduced as the complexity and the associated informational demands
increase. Uniform and consistent PPLT signal displays can reduce the level of informational complexity
placed on drivers. Driver expectancy is also an important consideration as signal displays that conflict with
prior experience increase complexity, informational demand, and driver error.
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OBJECTIVE
The objective of the photographic driver study summarized in this section is to evaluate the different
PPLT displays used in the United States. The evaluation explores driver understanding of the signal
indications under a variety of conditions. The conditions varied through the use of protected left-turn
indications, permitted left-turn indications, through-movement indications, and PPLT signal display
arrangements.
METHODOLOGY
The following tasks were undertaken to identify and evaluate the different types of PPLT signal displays
currently in use throughout the United States:
!
Conduct a literature review to become familiar with the findings of previous research;
! Select study locations and a method for conducting the study;
!
Develop study and data collection procedures;
! Administer the study; and
!
Reduce the data and analyze the results.
BACKGROUND
Many studies have been completed to establish warrants for the selection of the appropriate left-turn
strategy at a signalized intersection ( 14 , 15 , 45 , 46 , 47 , 48 , 49 , 50 ). A recent survey conducted by the
Western Section of ITE attempted to quantify the percentage of PPLT signal phasing used in the United
States ( 51 ). The survey results, obtained from 140 survey responses from 38 states, found that
approximately 25 percent of the 29,709 signalized intersections identified by survey respondents use PPLT
signal phasing.
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Signal Display Arrangements
Because of the many PPLT signal display arrangements and the potential for multiple applications, some
agencies have standardized their use. The Orange County Traffic Engineering Council, a consortium of
southern California communities, evaluated and selected a five-section cluster display for the overhead
PPLT signal display and a five-section vertical display for a far left pole-mount display ( 52 ). The purpose
of selecting a single set of displays was to implement traffic display uniformity in the region to eliminate or
at least minimize the problem with driver confusion.
A similar evaluation of left-turn traffic signal displays was completed in the state of Florida because of
the unique composition of tourist and retiree traffic on Florida's roadways ( 24 ). Florida concluded that
the five-section cluster display should be used for all PPLT applications. The policy statement states that
a five-section cluster display increases safety by reducing driver misunderstanding related to the permissive
left-turn movement. Kentucky has adopted a similar standard and has also selected the five-section cluster
display for use with PPLT ( 49 ).
A limited number of studies have compared the advantages or disadvantages of installing either the
horizontal, vertical, or cluster signal display arrangement. Similarly, the literature contains few studies that
evaluate the effect of the number of indications within each signal display arrangement. Nevertheless, the
studies that are described in the literature conclude that no significant difference in driver understanding
exists among signal display arrangements ( 23 , 26 , 53 , 54 ). Several of these studies are described below.
Noyce, Fambro, and Koppa conducted a study in Texas to evaluate drivers' understanding and
expectancy related to five-section horizontal, vertical, and cluster PPLT signal displays ( 26 ). A total of 469
completed surveys were received. When drivers were asked to identify where they would expect to find
each signal indication within a five-section horizontal display, only 9 percent of respondent expectancies
were consistent with the MUTCD requirements. Responses consistent with MUTCD requirements
increased to 28 percent with the five-section vertical arrangement and increased further increased to 57
percent with the five-section cluster arrangement. Therefore, the five-section cluster display was found to
be associated with the highest level of driver expectancy.
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Bonneson conducted a driver survey in Nebraska to evaluate driver understanding related to different
PPLT signal display arrangements ( 23 ). Analyzing the approximately 350 responses related to each signal
display, Bonneson found that there were no statistically significant differences between arrangements;
however, five percent more drivers were able to understand the horizontal arrangement than either the
vertical or cluster arrangements. Although there was no significant difference between arrangements,
Bonneson found that the permitted indication in horizontal and vertical arrangements were better
understood than in the cluster arrangement ( 23 ).
Plummer and King evaluated 40 West Virginia drivers and concluded that the five-section vertical
display resulted in best level of driver understanding ( 55 , 56 ). In contrast, a study by Agent in Kentucky
found that traffic signal displays in the five-section cluster arrangement were less confusing to left-turning
drivers ( 48 ). It is interesting to note that the Bonneson, Plummer, and Agent studies all identified different
five-section display arrangements as having the highest level of driver understanding.
Sobhi, analyzing data obtained by Ketron & Associates as part of a 1988 JHK study on left-turn signal
displays, found no significant difference in drivers' comprehension when considering arrangement ( 53 ). The
highest correct response rate was associated with separately located left-turn signal displays, including a
median-mounted five-section vertical display and a mast arm-mounted five-section cluster display. Both
of these displays were found to be superior to four-section vertical displays ( 17 ). In a study of drivers over
the age of 65, in New York, Maryland, and Virginia, Knoblauch found the highest level of driver
understanding related to the permitted signal indication in a vertical display ( 43 ).
Staplin completed a comprehensive study with the objective of measuring age-related differences in
drivers' ability to rapidly comprehend left-turn signal displays and supplemental signs, as the presentation
order varied ( 7 ). Fifty-four drivers participated in the study; 24 drivers between 19 and 49 years of age
and 30 drivers between 65 and 80 years of age. Staplin measured comprehension and response time to
both protected and permitted indications in several different arrangements under two conditions: 1) when
the display and supplemental sign are shown simultaneously and 2) when the supplemental sign is shown
five seconds before the display. The results of the comprehension study, showing the display configuration,
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display indication, supplemental sign, and percent of correct responses are presented in Table 1. Table
2 presents average response time results.
Staplin found little difference in driver comprehension of signal display arrangements due to age or
presentation order. When the number of correct responses were summed over all display/supplemental
sign combinations, the percent of correct responses were nearly identical for both the simultaneous
presentation of supplemental sign and display (68.6 percent) and the sign before display presentation
(68.2 percent). More pronounced differences were found when data were summarized by protected and
permitted responses as an 80 percent correct response rate was found with the protected indications and
56 percent correct response rate with the permitted indications.
Staplin found that the mean response time for the older test group consistently exceeded those for the
younger age group; however, the results were not significant. The most significant finding of the study was
the clear superiority in performance, by more than a full second, for both age groups when the supplemental
sign was presented five seconds before the display. This result was consistent with Alexander and
Lunenfeld's concept of positive guidance which among other things states that information should be spread
over space to reduce the drivers' informational load ( 57 ).
Staplin concluded that older and younger drivers reach a correct decision concerning the right-of-way
status conveyed by the signal display and supplemental sign significantly faster when the sign is presented
in advance of the display. Further, the significant difference in correct responses between protected and
permitted indications, combined with the slower response time of older drivers, may in part explain the
consistent over-representation of older drivers in crashes involving the left-turn maneuver at signalized
intersections.
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Table 1. Understanding of Display Types and Supplemental Signs in the Staplin Study.
Display
Indication1
Supplement.
Sign2
Percent Correct Responses
Together
Sign/Display
Age:
19-49
Age:
65-80
Age:
19-49
Age:
65-80
Protected
5-Sec. Cluster
GA/GB
R10-12a a
72.0
73.3
68.0
73.3
5-Sec. Cluster
GA/RB
R10-9 b
88.0
93.3
88.0
83.3
4-Sec. Vertical
GA/RB
R10-10 c
76.0
63.3
88.0
86.7
4-Sec. Horizontal
GA/GB
R10-5a d
84.0
90.0
92.0
80.0
4-Sec. Horizontal
GA/RB
R10-9
80.0
80.0
88.0
80.0
4-Sec. Vertical
GA
R10-12b e76.083.372.083.3
4-Sec. Cluster
GA
R10-10
96.0
93.3
96.0
93.3
3-Sec. Vertical
GB
R10-10
48.0
53.3
44.0
36.7
4-Sec. Cluster
GA/RB
R10-5a
92.0
76.7
88.0
76.7
4-Sec. Vertical
GA
Special f80.070.076.063.3
5-Sec. Vertical
GA/GB
R10-10
76.0
93.3
88.0
86.7
Permitted
4-Sec. Vertical
GB
R10-9
76.0
60.0
56.0
43.3
3-Sec. Vertical
GB
Special g44.053.340.043.3
4-Sec. Horizontal
GB
R10-12b
60.0
40.0
36.0
50.0
5-Sec. Horizontal
GB
R10-12 h
52.0
56.7
64.0
46.7
3-Sec. Vertical
FRB
R10-10
88.0
70.0
88.0
80.0
4-Sec. Vertical
FYB
Special f64.076.752.056.7
5-Sec. Cluster
GB
R10-12
20.0
33.3
68.0
56.7
1. G=Green; Y=Yellow; R=Red; A=Arrow; B=Ball; F=Flashing.
2. a - LEFT TURN SIGNAL -- YIELD ON (Green Ball)
b - PROTECTED LEFT ON GREEN ARROW
c - LEFT TURN SIGNAL
d - LEFT ON ARROW ONLY
e - LEFT TURN MUST YIELD ON (Green Ball)
f - LEFT TURN MUST YIELD ON FLASHING YELLOW
g - LEFT TURN NOT PROTECTED
h - LEFT TURN YIELD ON (Green Ball)
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Table 2. Response Time Average and Standard Deviation in the Staplin Study.
Age
Protected
Permitted
Together
Sign/Display
Together
Sign/Display
îµ½
σ
îµ½
σ
îµ½
σ
îµ½
σ
19-49
3.1
1.5
1.9
1.0
3.5
1.6
2.4
1.2
65-80
3.5
1.4
2.3
1.3
4.1
1.5
2.9
1.5
Regardless of the display arrangement selected, increasing the number of exposures to traffic signal
displays, specifically PPLT displays, has been shown to improve drivers' understanding of such displays.
Agent found only a small percentage of Kentucky drivers that did not understand the meaning of the five-
section cluster display after numerous exposures ( 48 ). Similarly, Perfater evaluated 10 intersections in
Virginia and found that more than one-third of the drivers surveyed were confused on the first encounter
with a permissive left-turn indication but only 12 percent remained confused after the second encounter
( 58 ). Both studies demonstrate the potential benefits associated with uniform application of PPLT signal
displays.
PPLT Signal Display Indications
The MUTCD requirement to simultaneously display two apparently conflicting signal indications during
the protected interval can be confusing to drivers. Asante evaluated the simultaneous use of green or red
ball indication and the green arrow indication in a five-section PPLT display in Texas ( 14 ). Field studies
were conducted at more than 100 sites and surveys were mailed to 6,000 Texas residents, of which 902
were returned.
Asante's data indicated a higher level of understanding when only the green arrow indication was
displayed as compared to when both the green ball and green arrow indications were displayed. When
the green arrow indication alone was compared to the simultaneous red ball and green arrow indications,
a larger significant difference in driver understanding was found. Asante concluded that a red ball and green
arrow should not be shown simultaneously on a five-section PPLT display. These results were confirmed
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by Bonneson's study in Nebraska as 75 percent of drivers understood the combined green arrow and red
ball indications while 85 percent of drivers understood the green arrow indication alone ( 23 ).
Green Arrow. The protected green arrow indication, as defined in the MUTCD, indicates that traffic
may "enter the intersection to make the movement indicated by such arrow" ( 59 ). Several studies have
looked at drivers' understanding of the green arrow.
Hulbert evaluated more than 3,000 drivers in a comprehensive study focusing on left-turn signal
indications ( 60 ). He found that only 51 percent of the drivers correctly interpreted the left-turn green
arrow. Hulbert concluded that some drivers appear to have difficulty understanding what maneuvers are
permitted or prohibited in response to signal arrows. Drivers' lack of understanding may result from the
complexity of the traffic signal control system rather than inadequate perceptual ability.
Hulbert also considered age effects in the comprehension of signal display indications and found that
drivers in the 55+ age group generally scored lowest while drivers in the 24 to 29 age group scored the
highest. Allen indicated a similar age-related finding in a study of symbol sign recognition ( 19 ). Benioff and
Rorabaugh found that approximately 87 percent of drivers had an acceptable understanding of the green
arrow indication, but older drivers had more difficulty and were incorrect more often ( 27 ).
Bonneson's study evaluated both the protected and permitted signal indication in the five-section
horizontal, vertical, and cluster displays ( 23 ). Approximately 115 responses were received for each
display/indication combination. The results of the study are summarized in Table 3. Bonneson found the
green arrow indication in the five-section cluster display had the highest level of driver understanding;
however, when the green arrow was shown without the corresponding through movement (ball) indication
in the five-section horizontal display, a slightly higher level of driver understanding was found.
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Table 3. Driver Understanding of Five-Section Signal Displays in the Bonneson Study.
PPLT Display
Arrangement
Through Movement
Arrangement
PPLT Signal
Display Placement
Percent Correct Response
Permitted Protected
Cluster
Vertical
Center LT Lane
66
80
Vertical
Vertical
Center LT Lane
80
82
Cluster
Vertical
Lane Line
66
85
Horizontal
Horizontal
Lane Line
76
53
Cluster
Horizontal
Lane Line
63
84
Horizontal1
Horizontal
Center LT Lane
80
88
1 Green arrow shown without corresponding through indication
Knoblauch evaluated drivers' understanding of both the protected and permitted signal indications,
considering a five-section vertical and cluster display, in Maryland, New York, and Virginia ( 43 ). The
results obtained from the 247 drivers who participated in this evaluation are shown in Table 4. Knoblauch
found that older drivers do not understand PPLT signal displays as well as younger drivers and neither
group had an acceptable level of understanding. The most common error related to the protected green
arrow indication, especially for older drivers, was the belief that they needed to yield or stop and look for
an acceptable gap in opposing traffic before turning left with the green arrow indication. The question
remains as to whether the PPLT signal display is deficient or is the comprehension level of the average
driver substandard?
Asante and Williams included an evaluation of protected and permitted signal indications in their Texas
study ( 14 ). The results are presented in Table 5. On average, 80 percent of Texas drivers correctly
understood the green arrow protected indication and 75 percent the green ball permitted indication when
presented in a five-section horizontal display. Results were improved when only the green arrow was
presented for the protected indication and when supplemental signs were added. When a three-section
vertical median-post-mounted display was used, approximately 50 percent of drivers did not understand
the meaning the green ball permitted display. Most of these drivers assumed the green ball indicated right-
of-way for the left-turn movement.
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Table 4. Percent of Drivers Understanding Signal Indications in the Knoblauch Study.
Signal
Indication1 Control
Display
Percent of Correct Responses
by Drivers' Age
< 65 Years Old
65 or Older
GA
Protected
5-Section Cluster
84 to 88
62 to 74
5-Section Cluster2
57
41
4-Section Vertical
93
79
GB
Permitted
5-Section Cluster
55 to 65
38 to 44
5-Section Cluster2
54 to 57
46 to 52
4-Section Vertical
62
52
1. G=Green; A=Arrow; B=Ball.
2. Included R10-12 Supplemental Sign
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Table 5. Driver Understanding Results from the Asante/Williams Study.
PPLT Signal
Indication1
Through
Indication1Supp. Sign2Display
Number of
Responses
Percent
Incorrect
GA/RB
GB
No
5-Section
79
23
GA
GB
No
5-Section
93
13
GA/GB
GB
No
5-Section
80
19
GA
RB
No
5-Section
80
20
GA/GB
GB
a
5-Section
91
8
GA/RB
RB
b
5-Section
93
34
GA
GB
c
5-Section
93
18
GA
RB
d
5-Section
86
5
GA
RB
No
3-Section
96
9
GA
GB
No
3-Section
95
27
GA
RB
b
3-Section
103
17
GA
GB
e
3-Section
103
31
GA
RB
b
3-Section
92
14
GA
GB
b
3-Section
69
23
GB
GB
No
5-Section
79
25
GB
RB
No
5-Section
93
34
GB
RB
f
5-Section
92
14
GB
GB
g
5-Section
86
24
GB
GB
No
3-Section
84
50
GB
RB
No
3-Section
107
47
1. G=Green; Y=Yellow; R=Red; A=Arrow; B=Ball; F=Flashing.
2. a - PROTECTED LEFT ON GREEN ARROW
b - LEFT TURN SIGNAL
c - PROTECTED LEFT TURN ON ARROW ONLY
d - PROTECTED LEFT TURN ON GREEN ARROW ONLY
e - NO TURN ON RED
f - LEFT TURN YIELD ON GREEN BALL
g - LEFT TURN PROTECTED ON ARROW ONLY
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Green Ball. The permitted green ball indication, as defined in the MUTCD, indicates that traffic may
"proceed straight through or turn right or left except as such movement is modified by signs, markings or
design" ( 59 ). This definition indicates that drivers facing a green ball indication have the right-of-way to
proceed, the given meaning of the green indication since traffic signals were first introduced in the early
1900s. The MUTCD definition continues with the following statement ( 59 ): "...but, vehicular traffic,
including vehicles turning right or left, shall yield the right-of-way to other vehicles, and to pedestrians
lawfully within the intersection or an adjacent crosswalk, at the time such signal indication is exhibited." It
is this caveat that allows the green ball indication to be used in each signal display arrangement for permitted
left-turn control.
The apparent inconsistency in the definition of the green ball indication has led to problems in driver
understanding since the inception of PPLT signal phasing ( 27 ). Staplin and Fisk found that the green ball
permitted indication was one of the most problematic as drivers were expected to interpret it as a signal
not to proceed into the intersection when their previously learned automatic response to green was an
assumption of right-of-way ( 5 ). When the green ball permissive indication was presented with the sign
'LEFT TURN YIELD ON GREEN (ball) in the Staplin study, 13 percent of younger drivers and 44
percent of older drivers indicated that they thought they had the right-of-way. Similarly, when the green
ball indication was presented with the sign "PROTECTED LEFT ON GREEN ARROW," 30 percent of
younger drivers and 64 percent of older drivers assumed that they had the right-of-way. A study by Allen
also demonstrated that the green ball permitted indication resulted in the longest driver response time and
lowest percentage of correct responses ( 61 ).
In the Knoblauch study presented in Table 4, the most common error with the green ball permitted
indication was the belief that drivers were required to stop before selecting a gap in opposing traffic. The
most dangerous misunderstanding of the green ball permitted indication was drivers who believed that the
green ball was a protected indication. Between seven and 20 percent of drivers over the age of 65 and
between four and 14 percent of drivers under 65 said they could turn left without stopping or yielding when
facing the green ball indication. Similar results were found in the 1975 Benioff and Rorabaugh study, where
81 percent of drivers understood the meaning of the left-turn green ball indication; however, seven percent
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of drivers believed that opposing traffic should yield to them during the permissive phase ( 27 ). Further, 12
percent of drivers thought it was illegal to turn left on the green ball indication.
Using data obtained by Ketron & Associates, Freedman and Gilfillan found the green ball indication
to be correctly understood by approximately 50 percent of the 180 drivers evaluated in Philadelphia,
Pennsylvania; Seattle, Washington; Dallas, Texas; and Lansing, Michigan ( 17 ). The green arrow in
combination with the green ball had a 70 percent correct response rate. No significant difference in results
was found between drivers with normal vision and drivers with color deficient vision. In most cases, the
differences in location were significant. When supplemental signs were added, driver understanding
decreased. Table 6 summarizes the results of this study.
In a study of older drivers, Staplin found that the most problematic indication was the permitted green
ball ( 5 ). To respond appropriately to the permitted green ball, older drivers depended on their inhibition
of previously learned responses. A signal element typically associated in the drivers' experience with go
was incorporated into a traffic control display requiring another, conflicting behavior. At the very least,
correct interpretation of the green ball indication appears to be slower and to increase demand on real-time
information-handling capacity, consistent with the results of the Allen study ( 61 ).
One of the biggest concerns with left-turn displays, expressed by both traffic engineers and drivers,
continues to be the ambiguous meaning of the green ball indication. Plummer and King hypothesized in the
early 1970s that left-turn signal displays did not convey their intended message equally ( 55 ). This
hypothesis was supported by their study as the 14 signal displays showing the green ball permitted
indication were not uniformly understood by drivers. In contrast, a later study by Noel suggested that the
ambiguity of left-turn displays may be highly exaggerated ( 62 ). Nevertheless, the literature supports the
concern of many traffic engineers—drivers may wrongly interpret the permitted green ball indication to
mean that the left-turn has the right-of-way, creating a potentially serious safety problem. It is this concern
that has led to the development of several unique indications for permitted left-turns.
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Table 6. Driver Understanding Results from the Ketron Study.
Display
Place
Left-Turn
Indication1
Through
Indication1
Driver Understanding (%)
Phil. Dallas
Seattl
e
Lansing
5-Section
Cluster
Lane
Line
GB
GB
34
71
59
48
GA/GB
GB
51
76
76
58
GA/RB
RB
68
78
61
54
5-Section
Cluster
Center
Line
GB
GB
32
69
61
44
GA/GB
GB
55
78
76
52
GA/RB
RB
57
60
44
38
5-Section
Vertical
Lane
Line
GB
GB
38
66
63
52
GA/GB
GB
64
76
63
56
GA/RB
RB
55
68
58
46
5-Section
Vertical
Pole
Median
GB
GB
47
69
56
50
GA/GB
GB
55
75
76
58
GA/RB
RB
62
75
51
42
4-Section
Vertical2
Lane
Line
GA/GB
GB
64
80
73
63
GA/RB
RB
64
70
51
38
4-Section
Vertical3
Center
Line
GA
GB
57
78
71
63
FYB
GB
28
47
61
46
4-Section
Cluster4
Center
Line
GA
GB
62
82
81
58
FRA
GB
0
0
0
0
3-Section
Vertical5
Center
Line
GA
RB
72
85
85
69
FRB
GB
51
66
73
63
1. G = Green; Y = Yellow; R = Red; A = Arrow; B = Ball; F = Flashing.
2. Bi-modal Arrows.
3. Seattle display.
4. Delaware display.
5. Michigan display.
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Variations in the Permitted Signal Indication
Because of the perceived problems with the green ball permitted indication cited in the previous section,
traffic engineers in California, Delaware, Maryland, Michigan, Nevada, and Washington have implemented
unique permitted indications in PPLT signal displays. Instead of the MUTCD green ball permitted
indication, these states have incorporated flashing permitted indications using either a red ball, red arrow,
yellow ball, or yellow arrow. The MUTCD requirement for simultaneous display of the through and left-
turn movement indication in the PPLT signal display is not consistently applied.
Flashing Red Ball and Arrow. Michigan has between 30 and 40 installations of the flashing red ball
permitted indication, mostly in urban areas with high-volume roadways. Requiring drivers to stop before
completing a permitted left-turn is believed to increase left-turn safety. Drakopoulos found the Michigan
three-section display with the flashing red ball permitted indication to be better understood by older drivers
than a four-section vertical display with a green ball permitted indication ( 44 ). Ketron reported that the
Michigan display performed equal to or better than the five-section vertical or five-section cluster display
using the permissive green ball indication ( 17 ).
The flashing red arrow permitted indication is used at three locations in Cupertino, California, at 13
locations in Maryland, and at 40 locations in Delaware ( 63 ). Vehicles are permitted to turn left on a
flashing red arrow indication after stopping and yielding to opposing traffic. A study in Maryland found that
62 percent of drivers correctly stopped before turning left when the flashing red arrow indication was
shown.
In response to arrow indications, studies have shown that drivers have trouble understanding what
maneuvers are permitted or protected with older drivers having more trouble than younger drivers ( 4 , 43 ,
54 ). JHK & Associates noted a number of potentially unsafe interpretations of the flashing red arrow
indication in PPLT operations ( 54 ). Analysis of crash data indicated a disproportionate rate of head-on
and angle collisions associated with flashing red arrow indications when compared to flashing red ball
indications. Ketron & Associates evaluated the Delaware flashing red arrow permitted indication and
found that none of the 180 respondents correctly understood the meaning of this indication ( 17 ).
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Knoblauch found that 58 percent of drivers under 65 years of age and 36 percent of drivers over 65
years of age correctly responded to a flashing red arrow permitted indication. This difference between ages
was statistically significant. Most of the drivers who responded incorrectly indicated that they would stop
and remain stopped at the flashing red arrow. Knoblauch also evaluated the flashing red ball indication
and found that 59 percent of drivers under 65 and 47 percent of driver over 65 years of age correctly
understood this indication. Again, most drivers who responded incorrectly interpreted this indication to
mean "stop and remain stopped."
There are mixed opinions concerning the use of red turn arrows because of the apparent ambiguity of
the red color meaning "stop" and the shape of the arrow meaning "go"; however, in the 1983 study by
Noel, analysis of 17,900 signal cycles showed that the red arrow indication accounted for significantly
fewer violations than the red ball indication and appeared to be more effective for protected turn control
at T-type intersections ( 62 ). Similarly, when used in equivalent traffic signal control settings, there was no
evidence that the red arrow indication is less safe or more ambiguous than the red ball indication ( 4 ).
Flashing Yellow Ball and Arrow. By MUTCD definition, the flashing yellow ball indication instructs
drivers to proceed through the intersection with caution ( 59 ). A flashing yellow arrow indication has the
same meaning, except that it applies only to drivers intending to make the movement indicated by the
arrow. The flashing yellow ball and arrow indications are used in the state of Washington and the Reno,
Nevada area.
In Washington, the change to a flashing yellow ball permitted indication was started in 1966 ( 64 ). The
objective of the flashing yellow ball was to create an indication that was intuitively obvious, conveying the
left-turn driver's obligation to yield. Additional application of the flashing yellow ball indication resulted
from a permitted indication safety study by Washington section of ITE between 1978 and 1985 ( 64 , 65 ).
The ITE committee evaluated a number of factors, including conflict rates, crash rates, drivers' opinions,
vehicle delay, and compliance at 30 locations. The findings of that study are summarized below:
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! The difference in color from green to yellow provides additional information and reduces the
chance that a driver will not distinguish the change from the protected green arrow interval to
the permitted indication;
!
The flashing yellow provides a "proceed with caution" message;
! The flashing yellow operation is used to further inform the driver of the change in right-of-way;
!
The flashing yellow ball permitted indication is safer than or as safe as the green ball indication
for an exclusive left-turn lane with a separate signal display indication;
!
The flashing yellow ball permitted indication is the best means to solve left-turn safety needs at
closely spaced intersections;
!
More drivers recognize that a flashing yellow ball permitted indication requires a driver to yield
before executing the turn movement. Drivers can confuse the green ball indication to mean the
same as the green arrow indication in PPLT phasing; and
! The flashing yellow ball permitted indication results in better compliance with pedestrian and
traffic laws than the permitted green ball.
Specific results of the study are presented in Table 7.
Couples evaluated the flashing yellow ball permitted indication at 88 intersection approaches in the
Seattle, Washington area ( 64 ). Couples results were consistent with previous studies conducted in
Washington, concluding that the flashing yellow ball permitted indication produced a lower average left-turn
crash rate than the green ball permitted indication. When considering only nighttime crashes, the differences
in crash rates were significant.
Table 7. Benefits of Flashing Yellow Ball Permitted Indications - Washington.
Measure of Effectiveness
Flashing Yellow
Green Ball
Average Number of Left-Turn Conflicts (per day)
3.14
5.00
Collisions per Million Vehicles
0.49
0.89
Percent of Drivers Who Consider the Display Safer
66%
34%
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One of the arguments against the use of flashing permitted indications is the arcade effect , which
occurs when drivers are able to see several intersections along an arterial simultaneously. To overcome
this problem and attenuate the visual impact of the flashing indications, Washington began using a lower
wattage lamp (67 watts) in the 12-inch signal lenses ( 64 ). The lower wattage lamp reduced the visual
impact of the flashing displays on through-movement drivers.
Several additional studies have been completed related to flashing yellow permitted indications.
Knoblauch's study found that 47 percent of drivers under the age of 65 and 44 percent of drivers over 65
years of age correctly understood the flashing yellow ball permitted indication ( 43 ). Approximately 40
percent of the wrong responses for each age group indicated the belief that drivers were required to stop
before entering an acceptable gap in the opposing traffic stream. Also, 12 percent of younger drivers and
16 percent of older drivers indicated that they would stop and remain stopped at the flashing yellow ball
indication. The majority of drivers who did not understand the flashing yellow ball indication indicated that
they treat the flashing yellow ball as a stop sign. Although stopping is a fail safe error, it does increase the
potential for rear-end crashes.
An important issue with the use of the flashing yellow permitted indications is how to display the
clearance interval. Conflicting yellow indications can lead to an increase in both driver misunderstanding
and response time ( 23 ). Yellow indications in permitted situations seems to give an ambiguous indication
to drivers. Bonneson found that a large number of drivers anticipated a red ball indication to follow a
yellow indication. Thus, driver expectancy may be violated with flashing yellow permitted indications.
Additional Research on Flashing Signal Indications
The primary objective of flashing permitted indications is to reduce signal display complexity; however,
Thackray and Touchstone, along with many traffic engineers, believe that flashing permitted indications do
not meet this objective ( 66 ). Many believe that flashing indications should only be used as a warning signal
because of the potential distraction and confusion caused by such an attention-demanding signal. In
contrast, a growing number of traffic engineers believe the flashing permitted indications can improve left-
turn safety. Several studies have investigated this issue.
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Staplin suggested the use of flashing yellow or red permitted indications as a method to ameliorate some
problems experienced by older drivers in left-turning situations, including insensitivity to vehicle approach
speed and slow reaction times( 67 ). Drakopoulos found that older drivers were less likely to misinterpret
the meaning of the left-turn permitted indication when a flashing indication was used; however, older drivers
were more likely to stop at a flashing yellow indication, even though stopping is only required with the
flashing red ( 44 ). Perfater found that only 5 percent of respondents were in favor of a flashing indication
to overcome the difficulty in interpreting the permitted green ball indication ( 58 ).
Sohbi used the Ketron & Associates data (Table 6) to evaluate drivers' understanding of permitted
indications used in different geographical regions of the U.S. ( 53 ). Data from 180 drivers were obtained
at study sites in Dallas and Philadelphia (green ball permitted indication), Seattle (flashing yellow ball
permitted indication), and Lansing (flashing red ball permitted indication).
When each permitted indication was evaluated in its home location, the green ball permitted indication
was correctly understood by 80 percent of drivers in Dallas and 72 percent of drivers in Philadelphia. The
flashing red ball permitted indication was correctly understood by 75 percent of drivers in Lansing. The
flashing yellow ball permitted indication was correctly understood by 76 percent of drivers in Seattle. A
significant difference in the level of driver understanding related to the various permitted indications was
found between geographic locations.
Sohbi also conducted a combined evaluation of driver understanding of the green ball, flashing yellow
ball, flashing red ball, and flashing red arr