Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

Test Track and Driving Simulator

Evaluations of Warnings to Prevent

Right-Angle Crashes at Signalized 

Intersections

Publication No. FHWA-HRT-08-070 OCTOBER 2008
FOREWORD 

The Federal Highway Administration (FHWA) Office of Safety Research and Development 

(R&D) is focused on improving highway operations and safety by increasing the knowledge and 

understanding of the effects of intersection design on operational efficiency and safety. 

Intelligent Transportation Systems (ITS) have been shown to have both safety and operational 

benefits. ITS is a worldwide initiative to incorporate communications and information 

technology in transportation systems. This study was conducted to investigate the potential for an 

ITS countermeasure to reduce the number of collisions that result from red-light violations at 

signalized intersections. In the United States, red-light violations result in about 1,200 fatalities 

or incapacitating injuries each year. At intersections where it is installed, a system that detects 

violations as they are about to occur and then warns all drivers who are at risk of a consequent 

collision could reduce the frequency of this type of fatal crash by as much as 88 percent. This 

research examined one of the assumptions underlying the estimated 88-percent crash reduction. 

 Michael F. Trentacoste 

 Director, Office of Safety 

Research and Development 

Notice 

This document is disseminated under the sponsorship of the U.S. Department of Transportation 

in the interest of information exchange. The U.S. Government assumes no liability for its 

contents or use thereof. This report does not constitute a standard, specification, or regulation. 

The U.S. Government does not endorse products or manufacturers. Trade and manufacturers’ 

names appear in this report only because they are considered essential to the object of the 

document. 

Quality Assurance Statement 

The Federal Highway Administration (FHWA) provides high-quality information to serve 

Government, industry, and the public in a manner that promotes public understanding. Standards 

and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its 

information. FHWA periodically reviews quality issues and adjusts its programs and processes to 

ensure continuous quality improvement. 
TECHNICAL REPORT DOCUMENTATION PAGE 

1. Report No. 

FHWA-HRT-08-070 

2. Government Accession No. 3. Recipient’s Catalog No. 

5. Report Date 

October 2008 

4. Title and Subtitle 

Test Track and Driving Simulator Evaluations of Warnings to Prevent RightAngle Crashes at Signalized Intersections 

6. Performing Organization Code 

7. Author(s) 

Vaughan W. Inman, Gregory W. Davis, Ihab El-Shawarby, and Hesham Rakha 

8. Performing Organization Report No. 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

FHWA Office of Safety R&D, Human Centered Systems Team 

Federal Highway Administration 

Turner Fairbank Highway Research Center 

6300 Georgetown Pike 

McLean, VA 22101 

11. Contract or Grant No. 

DTFH61-02-C-00064 

13. Type of Report and Period Covered 

Final Report, Oct 2002–May 2006 

12. Sponsoring Agency Name and Address 

Office of Operations Research, Development and Technology 

Federal Highway Administration 

Turner Fairbank Highway Research Center 

6300 Georgetown Pike 

McLean, VA 22101 

14. Sponsoring Agency Code 

15. Supplementary Notes 

This study was conducted by the FHWA Office of Safety R&D, Human Centered Systems Team. Funding for this research 

came from the U.S. DOT Joint Program Office. SAIC designed the research, analyzed the data, and prepared this report. 

AAI Engineering Support developed the simulation scenarios for Experiment 1 and ran the driving simulator. The Virginia 

Tech Transportation Institute collected data on the Virginia Smart Road and participated in analysis and reporting of that 

data. The research was performed for the FHWA Office of Operations R&D and the ITS Joint Program Office. Paul J. 

Tremont was the FHWA Task Manager and Thomas Granda was the FHWA contract manager. 

16. Abstract 

Two experiments (simulator and test track) were conducted to validate the concept of a system designed to warn potential 

victims of a likely red-light violator. The warning system uses sensors to detect vehicles that are unlikely to stop at red 

traffic signals and uses signs and flashing lights to warn drivers who might collide with a violator. Several human factors 

issues need to be addressed before such a system could be deployed. The experiments for this study addressed one of these 

issues—whether, if warned, a sufficient number of drivers would respond in a way that would allow them to avoid a rightangle collision. The results suggest that in the case where no other vehicles precede or follow, a majority of drivers who 

receive a conspicuous warning will act by braking sharply. Driver responses in both tests were similar. The test track results 

support the continued use of driving simulators in development of the system. Further research is required to assess 

responses to warnings given to drivers within a stream of traffic. 

17. Key Words 

Signalized intersection; red-light violation; warning; CICAS; 

perception-response time; intersection collision avoidance; 

mid-phase warning; vehicle-infrastructure integration; VII 

18. Distribution Statement 

No restrictions. This document is available to the public 

through the National Technical Information Service, 

Springfield, VA 22161. 

19. Security Classif. (of this report) 

Unclassified 

20. Security Classif. (of this page) 

Unclassified 

21. No. of Pages 

46 

22. Price 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized
ii 

SI* (MODERN METRIC) CONVERSION FACTORS 

APPROXIMATE CONVERSIONS TO SI UNITS

Symbol When You Know Multiply By To Find Symbol 

LENGTH 

in inches 25.4 millimeters mm 

ft feet 0.305 meters m 

yd yards 0.914 meters m 

mi miles 1.61 kilometers km 

AREA 

in2 square inches 645.2 square millimeters mm2

ft2 square feet 0.093 square meters m2

yd2 square yard 0.836 square meters m2

ac acres 0.405 hectares ha

mi2 square miles 2.59 square kilometers km2

VOLUME 

fl oz fluid ounces 29.57 milliliters mL 

gal gallons 3.785 liters L 

ft3 cubic feet 0.028 cubic meters m3 

yd3 cubic yards 0.765 cubic meters m3 

NOTE: volumes greater than 1000 L shall be shown in m3

MASS oz ounces 28.35 grams g

lb pounds 0.454 kilograms kg

T short tons (2000 lb) 0.907 megagrams (or "metric ton") Mg (or "t") 

TEMPERATURE (exact degrees) o

F Fahrenheit 5 (F-32)/9 Celsius oC 

or (F-32)/1.8 

ILLUMINATION 

fc foot-candles 10.76 lux lx

fl foot-Lamberts 3.426 candela/m2 cd/m2

FORCE and PRESSURE or STRESS 

lbf poundforce 4.45 newtons N 

lbf/in2 poundforce per square inch 6.89 kilopascals kPa 

APPROXIMATE CONVERSIONS FROM SI UNITS 

Symbol When You Know Multiply By To Find Symbol 

LENGTH

mm millimeters 0.039 inches in 

m meters 3.28 feet ft 

m meters 1.09 yards yd 

km kilometers 0.621 miles mi 

AREA mm2

 square millimeters 0.0016 square inches in2 

m2 square meters 10.764 square feet ft2 

m2 square meters 1.195 square yards yd2 

ha hectares 2.47 acres ac

km2 square kilometers 0.386 square miles mi2 

VOLUME 

mL milliliters 0.034 fluid ounces fl oz 

L liters 0.264 gallons gal 

m3 cubic meters 35.314 cubic feet ft3 

m3 cubic meters 1.307 cubic yards yd3 

MASS 

g grams 0.035 ounces oz

kg kilograms 2.202 pounds lb

Mg (or "t") megagrams (or "metric ton") 1.103 short tons (2000 lb) T 

TEMPERATURE (exact degrees) o

C Celsius 1.8C+32 Fahrenheit oF 

ILLUMINATION 

lx lux 0.0929 foot-candles fc 

cd/m2 candela/m2 0.2919 foot-Lamberts fl

FORCE and PRESSURE or STRESS 

N newtons 0.225 poundforce lbf 

kPa kilopascals 0.145 poundforce per square inch lbf/in2

*SI is the symbol for th International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E e 380. 

(Revised March 2003)
iii 

TABLE OF CONTENTS 

EXECUTIVE SUMMARY .......................................................................................................... 1

INTRODUCTION......................................................................................................................... 3

BACKGROUND ..................................................................................................................... 3

APPROACH............................................................................................................................ 3

Simulator Experiment ......................................................................................................... 4

Test Track Experiment........................................................................................................ 4

Dilemma Zone Behavior..................................................................................................... 4

DRIVING SIMULATION (EXPERIMENT 1).......................................................................... 5

DRIVING SIMULATOR CHARACTERISTICS ............................................................... 5

METHODS—DRIVING SIMULATION ............................................................................. 5

Conditions Tested ............................................................................................................... 5

Trigger Distances................................................................................................................ 7

Participants.......................................................................................................................... 8

Protocol............................................................................................................................... 8

Simulated Roadway ............................................................................................................ 9

Experimental Design........................................................................................................... 9

RESULTS—DRIVING SIMULATION ............................................................................. 14

Delay in Reaching Stop Line ............................................................................................ 14

Initial Speed ...................................................................................................................... 16

Accelerator Release Response Time................................................................................. 16

Brake Application Response Time ................................................................................... 18

Maximum Deceleration .................................................................................................... 18

DISCUSSION—DRIVING SIMULATION ....................................................................... 21

TEST TRACK (EXPERIMENT 2) ........................................................................................... 23

METHODS—TEST TRACK .............................................................................................. 23

Conditions Tested ............................................................................................................. 23

Trigger Distances.............................................................................................................. 24

Participants........................................................................................................................ 24

Protocol............................................................................................................................. 25

Test Road .......................................................................................................................... 27

Test Vehicle ...................................................................................................................... 28

RESULTS—TEST TRACK................................................................................................. 28

Initial Speed ...................................................................................................................... 28

Delay with 4-s Amber....................................................................................................... 29

Delay Attributable to Warning.......................................................................................... 30

Accelerator Release Response Time................................................................................. 32

Brake Application Response Time ................................................................................... 33

Maximum Deceleration .................................................................................................... 34

DISCUSSION—TEST TRACK .......................................................................................... 34

GENERAL DISCUSSION—DRIVING SIMULATION AND TEST TRACK.................... 37

CONCLUSIONS AND RECOMMENDATIONS FOR FURTHER RESEARCH .............. 39

REFERENCES............................................................................................................................ 41
iv 

LIST OF FIGURES 

Figure 1. Supplemental visual warning displays from the driving simulation ................................6

Figure 2. The simulated intersection with simulated in-road LEDs activated.................................7

Figure 3. Mean accelerator release times as a function of experimental condition and 

distance at which the phase change was triggered.........................................................................17

Figure 4. Gender by condition interaction in accelerator release time ..........................................18

Figure 5. Example 1 of data from Condition 3 participants who responded by stopping .............19

Figure 6. Example 2 of data from Condition 3 participants who responded by stopping .............20 

Figure 7. Warning configuration on the test track .........................................................................24

Figure 8. Virginia Smart Road intersection and the vehicle used for testing ................................27

Figure 9. Mean speed at the onset of the amber signals as a function of gender and amber 

trigger distance...............................................................................................................................29

Figure 10. The percent of drivers in the test track experiment who stopped in response to 

amber and warning onsets at 32 m (105 ft) and 55 m (180 ft).......................................................31

Figure 11. Cumulative percent of driver delays in reaching the stop line when the warning 

was triggered at 32 m (105 ft) (n = 29)..........................................................................................32

Figure 12. Mean accelerator release times to amber onset are shown as a function of grade 

and trigger distance........................................................................................................................33

Figure 13. Mean brake response time as a function of amber trigger distance and grade.............34

LIST OF TABLES 

Table 1. Trigger distances and metric equivalents...........................................................................8

Table 2. Signal change schedule for the first 32 intersections in the simulation...........................11

Table 3. Number of participants in experiment 1 as a function of experimental conditions 

(trigger distance and signal condition) and subject variables (gender and age group)..................13

Table 4. The percentage of participants delayed by a least 1 s in reaching the stop line shown 

as a function of age group, experimental condition, and warning trigger distance .......................15

Table 5. Percentage of participants delayed by at least 1 s in reaching the stop line shown as 

a function of experimental condition and warning trigger distance ..............................................16

Table 6. Percentage of participants that were delayed in reaching the stop line by 1 s or more 

shown as a function of condition ...................................................................................................16

Table 7. Time to stop line when warning is triggered, and required constant deceleration to 

stop at stop line assuming a 1 s delay before the deceleration begins ...........................................24

Table 8. Age and gender distribution of participants in the two warning distance conditions......25

Table 9. Amber signal trigger distances and their metric equivalents...........................................26

Table 10. The percent of trials in which participants stopped response to 4-s amber onset 

shown as a function of trigger distance..........................................................................................30
1 

EXECUTIVE SUMMARY 

The two experiments reported here were conducted to evaluate the feasibility of warning drivers 

who are at risk of a collision with a red-light violator. There are approximately 1,200 fatalities 

yearly as a result of right-angle collisions between red-light violators and vehicles that have the 

right of way. In the first of two experiments, a driving simulator was used to assess three 

alternative infrastructure-based victim warning configurations against a control configuration. 

The second of these experiments used a test track to validate one of the warnings that was 

simulated in the first experiment. 

The warnings evaluated in the simulator were (1) a half-second amber followed by a red signal, 

(2) an immediate change from a green signal to a red that was supplemented by red wig-wag 

signals and an electronic stop sign symbol and simulated strobe lights (all mounted on the traffic 

signal mast arm), and (3) mast arm warnings plus the onset of simulated in-pavement red-lightemitting diodes and simulated “intelligent” rumble strips. Driver responses to the three warning 

conditions were compared to responses in the control condition, the onset of a 4-s amber signal. 

Two-hundred and two drivers participated in the study, with approximately 50 drivers in each of 

the four conditions. 

Warnings were deemed to be sufficient to prevent a collision with a red-light violator if the 

arrival of the participant at the intersection was delayed by at least 1 s, per the arrival time that 

would be expected if they continued at the speed they were traveling when the warning was 

initiated. Based on the 1-s delay criterion, the most conspicuous warning, which included inpavement lights and intelligent rumble strips, theoretically prevented 67 percent of the potential 

collisions. That warning was more than twice as effective in preventing potential collisions as the 

control condition (4-s amber). 

A closed-road test track experiment was conducted to validate the findings from the driving 

simulation. The test track warning included an immediate red signal, wig-wag red lights, an 

electronic stop sign symbol, and strobe lights. Sixty drivers participated in the test track 

experiment. Half of these received the warning when 55 m (180 ft) from the intersection stop 

line, and half received the warning when they were 32 m (105 ft) from the intersection stop line. 

Ninety percent of the drivers who received the warning at 55 m (180 ft) stopped within 5 m 

(17 ft) of the stop line, while only one driver who received the warning at 32 m (105 ft) from the 

stop line stopped. The decelerations of the drivers that stopped for the warning were similar to 

those observed in the simulator. 

This study provides evidence that conspicuous infrastructure warnings might be effective in 

preventing collisions with red-light violators. In this study, no other vehicles were traveling 

either ahead or behind, and a majority of drivers braked aggressively for such a warning. 

However, important human factors questions remain pertaining to driver reactions to such 

warnings when other vehicles are ahead or behind and the effectiveness of infrastructure-based, 

vehicle-based, and combined infrastructure- and vehicle-based warnings. The Federal Highway 

Administration (FHWA) is currently conducting research to address these issues. 

3 

INTRODUCTION 

BACKGROUND 

At signalized intersections, straight crossing-path crashes occur when a vehicle violates a red 

light and collides with a vehicle crossing from an approach that has the right-of-way. The 

consequences of straight crossing-path crashes at signalized intersections are serious.(1) In 2006, 

there were 1,197 fatalities in 1,058 collisions of this type in the United States.(2) Zimmerman and 

Bonneson report that the median time into the red phase of the violations that result in collisions 

is about 9 s. In their sample of 41 red-light violation collisions, time-into-red ranged from 0.6 to 

44.2 s, with a mean of 14.1 s.(3) They suggest that the majority of red-light violation related 

collisions occur after queues have cleared. In this situation, both the signal violator and victim 

are likely to be traveling close to free-flow speed.

Ferlis described an infrastructure-based Intelligent Transportation System (ITS) countermeasure 

for reducing straight crossing-path crashes at signalized intersections.(4) He estimated that, where 

deployed, this infrastructure-based warning to both the violator and victim could reduce straight 

crossing-path collisions by as much as 88 percent. The Ferlis estimate was based on the 

assumptions that 80 percent of violators would respond if warned, and 40 percent of potential 

victims would respond if warned.1

 

The present research tested the Ferlis assumption that 40 percent of potential victims who are 

warned would respond to avoid a right-angle collision. Two experiments are reported—one 

based in a driving simulator, and one conducted on a closed-road course. 

APPROACH 

Two experiments were designed to test the assumption that 40 percent of potential victims would 

respond to a red-light violation warning. The first experiment used a driving simulator to assess 

three alternative victim warning configurations and a control configuration. The second 

experiment used a closed-road course to validate one of the warnings that was simulated in the 

first experiment. 

It is recognized that red-light violators can only be reliably detected when they are relatively 

close to an intersection. Ferlis suggested that a violator approaching an intersection at 56 k/h 

(35 mi/h) could be detected about 43 m (141 ft) from an intersection.(4) Because warnings to 

potential victims cannot occur earlier than detection of potential violators, it is possible that 

potential victims might be already committed to entering the intersection. Therefore, any victim 

warning needs to be compelling and immediately comprehensible. The warning system 

envisioned by Ferlis had three major elements: (1) sensors to detect the speed of vehicles 

approaching a red traffic signal, (2) an algorithm to compute the likelihood of an approaching 

vehicle to run a red light, and (3) a warning to potential violators and their potential victims. The 

 

 1 The 88-percent reduction is arrived at by adding the two probabilities (0.8 + 0.4) and subtracting their joint 

probability (0.8 × 0.4). 
4 

experiments were designed to assess the probability that selected warnings would induce 

potential victims to slow or stop when the warnings were presented at a point on the intersection 

approach where they had already committed to proceeding through an intersection without 

stopping, regardless of whether or not the signal remained green. 

SIMULATOR EXPERIMENT 

To assess the response to an unexpected warning, it is first necessary to create an expectation 

that the traffic signals are operating as usual. To build that expectation in the driving simulator, 

participants drove through 32 intersections before they were presented a warning. Upon 

approach to 25 of those 32 intersections, no slowing or stopping was required in response to 

signals that cycled back to green well before the participants needed to slow or stop, and none 

changed to amber when the participant was close to the intersection. Upon approach to the 

remaining four intersections, the signal changed from green, but the participant always had at 

least 168 m (550 ft) or 8.3 s to slow or stop. Thus, previous to the 33rd intersection, participants 

never needed to make a rapid decision in response to traffic signals. Furthermore, at the six 

intersections that immediately preceded the 33rd, the light turned green before the participant 

vehicle was within 122 m (400 ft) of the stop line. At the 33rd intersection, when they reached a 

trigger distance of either 55 m (180 ft) or 66 m (215 ft), participants were presented with one of 

the three warnings or with a 4-s amber (the control condition).2 

TEST TRACK EXPERIMENT 

To build an expectation of normal phase changes in the closed-road test track experiment, 

participants drove through the same intersection 24 times before a warning was presented. On 

20 of the 24 intersection approaches before the warning was presented, a 4-s amber was 

presented. Some of those phase changes occurred when the participant was less than 2 s from the 

stop line. On the remaining four approaches, a green signal was presented. Thus, in the simulator 

the intended expectation at the warning intersection was that of a green light. In contrast, in the 

closed road the intended expectation at the warning intersection was a high probability of a 

4-s amber signal. In both the simulation and in the field, the warning should have been 

unexpected. 

DILEMMA ZONE BEHAVIOR 

The dilemma zone, as defined here, is the area upstream of the stop line where drivers are 

inconsistent in stop-and-go decisions when confronted with the onset of an amber signal. 

Although the primary purpose of these studies was to examine driver response to unexpected 

warnings at signalized intersections, the 4-s amber presentations that occurred in proximity to the 

intersections also provided an opportunity to examine dilemma zone behavior.(5) The test track 

experiment findings with respect to dilemma-zone decisionmaking may be found in separate 

reports by El-Shawarby et al.(6,7,8) However, responses to amber-signal onsets are summarized 

briefly in the Results section of this report. 

 

 2 The 4-second amber-signal duration was selected as representative of the duration used by local (metropolitan 

Washington, DC, area) transportation departments for the class of road that was simulated. 
5 

DRIVING SIMULATION (EXPERIMENT 1) 

DRIVING SIMULATOR CHARACTERISTICS 

The simulator (as configured for this experiment) consisted of a late model compact car chassis 

that was mounted inside of a section of a cylindrical projection screen. The cylindrical screen 

had a vertical radius of 2.7 m (107 inches). When looking at the center of the projections, the 

driver’s eye point was set 20 cm (8 inches) back from the center of the cylinder. An Electrohome 

Marquee 9500LC projected a 1,920 by 1,200 pixel image onto the screen. The image covered the 

driver’s forward field-of-view 88 degrees horizontally 50 degrees vertically. A Silicon Graphics 

Infinte Reality 2 graphics subsystem generated the images at 60 Hz. The vehicle brake, 

accelerator, and steering wheel inputs were routed to a Silicon Graphics ONYX2 computer that 

hosted a vehicle dynamics model. The vehicle dynamics model updated the virtual world on a 

per frame basis. Key features of the vehicle dynamics model were as follows: 

• Six degree-of-freedom equations.3

• A simplified model of a four-speed automatic transmission. 

• Tire skid limits based on side friction and braking force. 

• Steering disturbances (to simulate side forces such as wind). 

• A 3-degree-of-freedom (tilt, roll, and heave) motion base that responded to vehicle 

dynamics model outputs. 

METHODS—DRIVING SIMULATION 

Conditions Tested 

Four signal conditions were tested in the simulator: three specially designed infrastructure-todriver warning displays and a conventional 4-s amber phase that served as a control condition. 

The three potential warning displays varied in conspicuity and in the degree to which they 

differed from current infrastructure. 

Four-Second Amber (Control) 

The control signal condition was a 4-s amber that was initiated when the distance of the 

participant vehicle to the stop line became equal to or less than to a particular trigger distance. 

The warning trigger distance was either 55 m (180 ft) or 66 m (215 ft). The difference between 

control condition responses and warning condition responses was intended to provide a measure 

against which responses to the warnings could be compared. 

 

 3 All six degrees of freedom were used in computer image generation. The motion base responded to a subset of 

these degrees of freedom and was subject to its own mechanical limitations. 
6 

Condition 1 

This signal condition consisted of a short amber (0.5 s) followed by a red. This condition was 

included because it could be implemented easily at low cost by using existing infrastructure. The 

distinguishing feature of this warning is the early onset of the red. Although this warning 

alternative has the advantage of a low installation cost relative to the other warnings, it is 

possible that the early red onset would not be compelling enough. Also, because the red is 

delayed 0.5 s relative to the other warnings, it may delay braking by up to 0.5 s and thus require 

even more aggressive braking than the other warnings. 

Condition 2 

The second warning condition was to immediately change the signal from green to red and to 

supplement the red signal lenses with additional signs and lights. Added red lenses (wig-wag 

lights) were situated on both sides of the normal red lens alternated at 1 Hz. This was combined 

with a variable message sign that was scaled to be 1 m (4 ft) on a side and was positioned 

between two signal heads, which were aligned with the approach lanes. When this otherwise 

black sign was activated, a large red hexagonal stop symbol was presented, and two expanding 

white spots simulated strobe lights in the lower corners of the sign. The spots alternated at 2 Hz. 

Figure 1 depicts a driver’s view of this warning. 

Figure 1. Illustration. Supplemental visual warning displays from the driving simulation. 

Condition 3 

The third warning condition included all the features from Condition 2, plus simulated intelligent 

rumble strips and simulated red light-emitting diodes (LEDs) along the lane markings. Intelligent 

rumble strips are devices that could be embedded in the roadway and would be detectable only 

when triggered by a collision warning system.(4) Intelligent rumble strips would produce the 

same auditory and tactile stimulation as the conventional rumble strips that are used as lane 

deviation and speed advisory warnings. Intelligent rumble strips were simulated by activating a 

1.9-cm (0.75-inch) vertical heave of the simulator’s motion base at 4 Hz for 1 s. The vertical 
7 

heave resulted in both haptic and auditory stimulation that closely approximated the sensations 

generated by a series of four speed advisory rumble strips. Because of a programming error, the 

intelligent rumble strips were triggered when the participant vehicle was 61 m (200 ft) from the 

intersection stop line regardless of the trigger distance for the visual warnings. The simulated inroad LEDs were placed every 6 m (20 ft) along both the left- and right-lane markings of the 

through lane and every 60 cm (24 inches) across the intersection stop line. The simulated LEDs, 

are shown in figure 2. 

Figure 2. Illustration. The simulated intersection with simulated in-road LEDs activated. 

Trigger Distances 

In addition to four signal conditions, two trigger distances were used for initiating the signal 

conditions: 66 m (215 ft) and 55 m (180 ft). Table 1 shows the two trigger distances and their 

metric equivalents. When the study began, the trigger distance for activating the warning and 

control conditions was 66 m (215 ft). Selection of this distance rested on an assumed perceptionreaction time of 1 s and an instructed travel speed of 72 km/h (45 mi/h). Under these 

assumptions, a complete stop at the stop line in response to a warning would require 0.65 g of 

constant deceleration. It seemed reasonable to assume that no participant would attempt to stop 

for the amber signal given that deceleration requirement. However, the assumption proved to be 

wrong. Of the first 11 participants in the control (4-s amber) condition, 4 participants came to a 

complete stop before crossing the stop line. They were able to do this because perceptionreaction times were less than 1 s and because decelerations peaked at 0.9 g (the maximum 

allowed by the vehicle dynamics model). Because such a high proportion of control condition 

participants were stopping, the trigger distance was reduced from 66 m (215 ft) to 55 m (180 ft). 

Assuming a 1-s response time, the 55-m (180-ft) trigger distance would require 0.85 g of 

constant deceleration. The warning trigger distance was 66 m (215 ft) for the first 

52 participants and 55 m (180 ft) for the final 162 participants. 
8 

Table 1. Trigger distances and metric equivalents. 

U.S. customary (ft) Metric (m) 

180 ft 55 m 

215 ft 66 m 

1 ft = 0.305 m 

Participants 

Participants were licensed drivers recruited from the Washington, DC, metropolitan area. 

Useable data were obtained from 202 of 214 participants. Twelve participants (5.6 percent) 

failed to complete the experiment because of simulator sickness symptoms. Two age groups 

(younger, 18 to 65 years of age; and older, 66 to 87 years of age) and gender were approximately 

balanced across the four warning conditions.

Protocol 

Participants reviewed and signed an informed consent document upon arrival at the research 

center. They were then asked a series of questions to screen for impairment by illness, drugs, or 

alcohol. Vision was tested to ensure that they met or exceeded a minimum visual acuity of 20/40 

in at least one eye. Next, participants were escorted to the driving simulator facility where they 

were asked to perform the Sharpened Romberg postural stability test and to complete the 

Simulator Sickness Questionnaire (SSQ).(9,10) The Sharpened Romberg and SSQ were completed 

before the participant entered the vehicle to provide baselines against which post-simulation 

health status could be assessed. 

Following completion of these tests, participants were seated alone in the Federal Highway 

Administration (FHWA) Highway Driving Simulator. A researcher read the following 

instructions aloud: 

The Federal Highway Administration provides guidelines and 

recommendations to the states regarding federally funded highway 

design and operations. One aspect of these guidelines has to do 

with the phasing of traffic signals. The purpose of this experiment 

is to evaluate various phasing patterns. 

You are asked to drive along a two-lane rural road that has a traffic 

signal every half mile. Drive as you normally would while 

maintaining 45 mph, which is the posted speed limit. Obey all 

traffic laws and observe all traffic signals. When you need to slow 

down or stop for a traffic signal, accelerate back to 45 mph as soon 

as you get a green light. Except when you are responding to a 

traffic signal, if you exceed 50 mph, or slow below 40 mph, the 

experimenter will remind you that you should be maintaining 

45 mph by saying “Watch your speed.” 
9 

Pay attention to how the signals change. At the conclusion of your 

drive, which should take about 25 minutes, we will ask you 

questions about the signals. Thank you, and drive safely. 

Simulated Roadway 

The simulated roadway was a contiguous series of 1.3-km (0.8-mi) segments of a straight and 

level two-lane rural highway. In the middle of each segment, the same intersection recurred. At 

this intersection, the road widened to include a dedicated left-turn lane. Although the same 

roadway segment recurred 33 times, the roadside scenery varied slightly from segment to 

segment. Bridges, guardrails, trees, and fences close to the roadway remained the same on each 

segment, but widely spaced buildings (e.g., churches and houses) varied between segments. 

Except for the absence of vertical curvature in the simulation, the roadway was modeled on US 

29 and the intersection of US 29 with State Route 234 in Manassas, VA. 

Experimental Design 

The research design was entirely between groups with only one observation per participant. 

Statistical power was achieved by the use of a relatively large participant sample size rather than 

by repeated measurement of the same participants. The between groups design was necessitated 

by the assumptions underlying the study. First, it was assumed that red-light violator warnings 

would be rare and that any individual might experience such a warning only once or twice over a 

driving career. If true, an individual’s response to repeated warnings would be of little or no 

interest. Second, it was assumed that, as with most new traffic control devices, currently licensed 

drivers would receive little or no information about an infrastructure-based warning device. 

Given these assumptions, it was deemed imperative that any red-light violator warning be 

intuitive and quickly comprehensible on first exposure and without the requirement of previous 

training or experience. 

As stated in the introduction, traffic signals previous to the final (warning) intersection were 

timed to build an expectation that the final signal would be functioning normally and would 

likely be green. Table 2 shows the signal functioning intended to build this expectation. 

Assuming a 1-s brake response time to the onset of an amber at any intersection before the 33rd, 

no decision to immediately brake would have required a deceleration of more than 1.8 m/s2 

(5.9 ft/s2), or 0.18 g. Because deceleration rates and braking latency were under participants’ 

control, the number of braking, stopping, and slowing events varied among participants. Based 

on the distance from the signal at amber onset and on the duration of the red, the table indicates 

the signals for which most participants came to a complete stop (one asterisk), and the signals for 

which no accelerator release or brake response was required to avoid braking at more than 

3 m/s2 (10 ft/s2), or 0.31 g (two asterisks). 

At the first two intersections, the signals remained green at all times. At all other intersections, 

there was an amber-phase onset. However, the amber and red phases occurred when the 

participant was far enough from the intersection that slowing or stopping was only required at 

seven intersections, which are indicated by a single asterisk in table 2. At the 33rd intersection, a 

4-s amber was triggered at 531 m (1,741 ft). The red phase that followed was 14 s. The light then 
10 

turned green. Assuming the participant maintained 72 km/h (45 mi/h), the vehicle would have 

been 169 m (553 ft) from the stop line when the light turned green. Given that up to that point, 

the signals had changed only once when the vehicle was closer than 169 m (553 ft) and that (at 

the instructed speed) the vehicle was only 8 s from the stop line at green onset, it was assumed 

that the participant would not be anticipating another phase change at the 33rd intersection. 
11 

Table 2. Signal change schedule for the first 32 intersections in the simulation. 

Intersection Distance from intersection 

at onset of amber 

Duration 

of red 

1 Always Green 0 

2 Always Green 0 

3 764* 11 

4 967 11 

5 1,043 11 

6 1,075 11 

7 1,092 11 

8 1,102 11 

9 1,102 11 

10 433* 15 

11 639* 15 

12 1,036 11 

13 1,049 11 

14 1,075 11 

15 986 11 

16 1,042 11 

17 770* 12 

18 1,127 12 

19 1,082 12 

20 1,056 12 

21 1,239 18 

22 1,423* 18 

23 961* 18 

24 1,397 18 

25 1,043* 18 

26 1,269 10 

27 1,334** 10 

28 1,400** 10 

29 1,469** 10 

30 1,538** 10 

31 1,603** 10 

32 1,672** 10 

33*** 1,741** 14 

1 ft = 0.305 m 

* Most participants stopped, others slowed. 

** Could have proceeded without slowing. 

*** Warning triggered after change back to green. 

 
12 

Independent Variables 

Four between group variables were tested. Signal condition had four categories, and the other 

variables each had two categories: 

• Signal condition: 

o Control—4-s amber. 

o Condition 1—0.5-s amber. 

o Condition 2—wig-wag lights, stop symbol on variable message sign, and simulated 

strobes. 

o Condition 3—Condition 2 plus in-pavement LEDs and intelligent rumble strips. 

• Trigger distance: 

o 55 m (180 ft). 

o 66 m (215 ft). 

• Age groups: 

o Under 65 years of age. 

o 65 years of age or older. 

• Gender: 

o Male. 

o Female. 

Dependent Measures 

The dependent measures in this study were as follows: 

• Delay in reaching stop line. 

• Initial speed. 

• Accelerator release time. 

• Brake response time. 

• Maximum deceleration. 

Table 3 summarizes the experimental design and shows the number of participants within each 

cell. 
13 

Table 3. Number of participants in experiment 1 as a function of experimental conditions 

(trigger distance and signal condition) and subject variables (gender and age group). 

Females > 65 Males > 65 Females < 66 Males < 66 

180 ft Control 10 9 11 7 

180 ft Condition 1 8 10 10 11 

180 ft Condition 2 8 12 9 10 

180 ft Condition 3 9 8 8 10 

215 ft Control 2 1 3 5 

215 ft Condition 1 2 1 5 4 

215 ft Condition 2 2 2 5 4 

215 ft Condition 3 2 4 6 4 

1 ft = 0.305 m 

Delay in Reaching Stop Line 

Because the purpose of the warnings is to prevent drivers from being in the intersection when a 

red-light violator is crossing their path, it is critical that the warning delay their arrival to the 

intersection until the threat has passed. To evaluate warning effectiveness, a violator was 

assumed to be traveling at the same speed as the participant and to have been detected at the 

warning trigger distance (i.e., either 66 m (215 ft) or 55 m (180 ft)). Thus, if both vehicles 

proceeded at the same speed they were traveling when the violator was detected, then they would 

collide in the intersection. To allow some leeway for variation from the assumed scenario, a 

minimum of 1-s arrival delay of the potential victim (participant driver) was deemed to represent 

a success. If the violator continued at 72.5 km/h (45 mi/h), a 1-s delay in arrival of the participant 

would result in the participant passing 20 m (66 ft) behind the violator. The delay in arrival was 

calculated from each participant’s actual speed, rather than the instructed speed. The amount of 

delay in seconds was then converted to a binary variable: 0 if less than 1 s, 1 if equal to or greater 

than 1 s. 

Initial Speed 

Although the delay in reaching stop line variable was calculated based on initial speed, the 

decision to delay or continue at the current speed could be influenced by the driver’s speed when 

the warning is initiated. Because the warning was triggered based on distance, not time to 

intersection, lower speeds would allow lower deceleration rates, longer response times, or both. 

Therefore, initial speed was analyzed for possible influences on the other results. 

Accelerator Release Time 

Accelerator release time was measured because it provides the first available indication that the 

warning stimulus has been detected. Accelerator release time is sometimes referred to as 

perception time.(10) 
14 

Brake Response Time 

Brake response time is the most frequently used measure of driver latency to react to the onset of 

amber signals. In the traffic engineering literature, brake response time is often referred to as 

perception-reaction time.(12) In field studies, brake response time is often measured from the 

onset of an amber signal to the onset of the vehicle brake lights (e.g., Wortman and Matthias, 

1983; and Gates et al., 2007).(5) However, in this study, the beginning of brake pedal deflection is 

used as the terminator for brake response time. 

Maximum Deceleration 

Maximum deceleration was measured to suggest the intensity of the braking that followed the 

brake responses to the signal conditions. It was the largest frame-to-frame deceleration that was 

recorded between the onset of the amber/warning signal and crossing the stop line. It was only 

recorded for participants for whom a brake response time was recorded. 

RESULTS—DRIVING SIMULATION 

Delay in Reaching Stop Line 

The primary measure of a collision warning’s effectiveness is whether or not a collision is 

avoided. The hypothetical threat for which the signal conditions were triggered was a red-light 

violator who approached the intersection on the cross street. A 1 s or greater delay of the 

participant in reaching the intersection, based on the participant’s speed when the warning was 

initiated, was counted as an avoided collision. 

A logit model was used to analyze the delay results. A logit analysis was used because the 

independent variables were classification variables and the dependent variable (delay) was 

binary. Although logit analysis is similar to the more familiar analysis of variance in that it 

includes both main effects and interactions, it differs in the way statistical significance is 

evaluated. Rather than providing statistical significance for each effect, the model as a whole is 

evaluated, with effects dropped from the model until the model no longer accounts for the 

observed variability. In a logit analysis, the best fitting model is the most parsimonious model for 

which the goodness-of-fit does not differ significantly from the full model.(13,14) That is, variables 

and interactions are dropped from the model until a model with the fewest variables that still 

enables reconstruction of the obtained pattern of results is identified. The dependent measure was 

the 1-s delay (yes or no). The independent variables in the full (saturated) logit model were age, 

gender, signal condition, trigger distance, and all interactions of these variables. The final model 

included signal condition, age, trigger distance, and the interaction of trigger distance with signal 

condition. Table 4 shows the results as a function of the variables in the best fitting model. Older 

drivers were more likely to be delayed regardless of trigger distance or warning condition. 
15 

Table 4. The percentage of participants delayed by a least 1 s in reaching the stop line 

shown as a function of age group, experimental condition, and warning trigger distance. 

Age Group Trigger 

Distance

Signal 

Condition Delayed 

Under 65 180 ft Control 17% 

Under 65 180 ft Condition 1 17% 

Under 65 180 ft Condition 2 48% 

Under 65 180 ft Condition 3 47% 

Under 65 215 ft Control 38% 

Under 65 215 ft Condition 1 60% 

Under 65 215 ft Condition 2 78% 

Under 65 215 ft Condition 3 89% 

65 or Over 180 ft Control 37% 

65 or Over 180 ft Condition 1 71% 

65 or Over 180 ft Condition 2 61% 

65 or Over 180 ft Condition 3 70% 

65 or Over 215 ft Control 67% 

65 or Over 215 ft Condition 1 67% 

65 or Over 215 ft Condition 2 100% 

65 or Over 215 ft Condition 3 100% 

1 ft = 0.305 m 

However, it is wrong to infer the greater effectiveness of the warning with older adults is directly 

attributable to age. As reported below, older adults were, on average, traveling slower when the 

warnings came on. Initial speed can also predict delay outcome. Indeed, when initial speed is 

entered as a covariate in the logit model, age and the interaction of trigger distance with signal 

condition can be dropped from the model with no significant loss in prediction. Table 5 shows 

the percent of participants delayed as a function of the two variables that remained significant 

after accounting for initial speed—signal condition and trigger distance. 

The warning treatments were more effective at 66 m (215 ft) than at 55 m (180 ft). The greater 

the conspicuity of the warning, the greater the proportion of participants who delayed. 
16 

Table 5. Percentage of participants delayed by at least 1 s in reaching the stop line shown as 

a function of experimental condition and warning trigger distance. 

Trigger 

Distance 

Experimental 

Condition Delayed 

180 ft Control 27% 

180 ft Condition 1 43% 

180 ft Condition 2 54% 

180 ft Condition 3 59% 

180 ft All Conditions 46% 

215 ft Control 45% 

215 ft Condition 1 63% 

215 ft Condition 2 83% 

215 ft Condition 3 92% 

215 ft All Conditions 71% 

1 ft = 0.305 m 

Table 6 shows the percentage of participants delayed without regard to trigger distance. 

Table 6. Percentage of participants that were delayed in reaching the stop line by 1 s or 

more shown as a function of condition. 

Signal Condition Delayed 

Control 31% 

Condition 1 49% 

Condition 2 61% 

Condition 3 67% 

Initial Speed 

Among the younger adults, the instruction to maintain 72 km/h (45 mi/h) was successful. At the 

beginning of the final phase change event (i.e., onset of the warning or amber signal), the mean 

speed of the younger adult groups was 71.5 km/h (44.4 mi/h) with a standard deviation of 

6.6 km/h (4.1 mi/h). The older adults tended to drive below the instructed speed. They had a 

mean speed at the beginning of the final phase change event of 66.8 km/h (41.5 mi/h) with a 

standard deviation of 7.4 km/h (4.6 mi/h). Initial speed was not significantly related to any of the 

other independent variables (all p > 0.17). 

Accelerator Release Response Time 

Of the 202 participants, 156 responded with accelerator releases between the onset of the 

warning/amber phase and arrival at the stop line. Of these 156 accelerator releases, 4 response 

times were deemed to be outliers and excluded from further analyses. There were no outliers 

with extreme short responses. Long response outliers were identified as responses that were more 

than 2 s. Those responses were more than 1.5 times the interquartile range from the upper edge 

of that range. The interquartile range is defined as the distance between the 25th-percentile 
17 

response time and the 75th-percentile response time. Of the four response times deemed outliers, 

one came from an older adult and three came from younger adults. Assuming a 72 km/h 

(45 mi/h) approach speed, 2 s after the onset of the warning participants would have been 25 m 

(83 ft) or 15 m (48 ft) from the stop line, depending on whether the trigger distance was 66 m 

(215 ft) or 55 m (180 ft), respectively. 

A full factorial analysis of variance was conducted on the 152 remaining accelerator release 

times. Signal condition, age group, gender, and trigger distance were the independent variables. 

The significant main effects are represented in figure 3 where it can be seen that responses at the 

66-m (215-ft) trigger distance were faster than at the 55-m (180-ft) trigger distance, 

F (1, 120) = 3.7, p < 0.05, and that responses to the 0.5-s amber were longer than to the other 

conditions, F (3, 120) = 12.7, p < 0.01. 

0

0.2

0.4

0.6

0.8

1

1.2

180 215

Trigger Distance (ft)

Mean Throttle 

Response Time (s)

Control Condition 1 Condition 2 Condition 3

1 ft = 0.305 m 

Figure 3. Line graph. Mean accelerator release times as a function of experimental 

condition and distance at which the phase change was triggered. 

Although the warning condition effect was significant, it was solely attributable to the male 

drivers. As seen in figure 4, gender and experimental condition interacted, F (3, 120) = 3.9, 

p < 0.05, such that females showed almost no difference in response time across the four warning 

conditions. 
18 

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Control Condition 1 Condition 2 Condition 3

Experimental Condition

Mean Throttle 

Response Time (s)

Female Male

Figure 4. Bar graph. Gender by condition interaction in accelerator release time. 

The above accelerator release times were not conditional on whether the participant subsequently 

braked or came to a stop. When only the accelerator releases of participants who subsequently 

were delayed by at least 1 s are considered, then the results are quite different. In the case of 

these 86 participants, the only significant factor was age, F (1, 56) = 7.1, p < 0.05, where drivers 

in the older group began their accelerator release an average of 0.16 s later than drivers in the 

younger group. 

Brake Application Response Time 

Initial analyses of the brake response times indicated a number of outliers, as defined in the 

preceding section. Exclusion of participants with brake response times greater than 2.2 s or less 

than 0.3 s reduced the sample of brake response times from 167 to 159. A full factorial analysis 

of variance that included condition, age group, trigger distance, and gender as variables indicated 

there were no statistically reliable effects. Analysis of variance that included only signal 

condition as a variable also failed to reveal any effects to be significant at the 0.05 level. Overall, 

the mean brake response time was 1.21 s with a standard error of 0.02 s. 

Maximum Deceleration 

Participants who were delayed in reaching the stop line by at least 1 s had a mean for maximum 

deceleration of 0.78 g (standard error = 0.01). Figure 5 and figure 6 show plots of the 

deceleration and braking behavior of two participants from Condition 3 who responded by 

stopping following the warning triggered at 55 m (180 ft) from the stop line. The top panel 

shows data from a 19-year-old participant who appeared to hesitate before releasing the 

accelerator and hesitated again before exerting near maximum braking force. The lower panel 
19 

shows data from a 66-year-old participant who showed less hesitation in exerting maximum 

braking and then coasted to the stop line. 

0

20

40

60

80

100

120

140

160

180

200

0.000

0.467

0.933

1.400

1.866

2.333

2.800

3.266

3.733

4.200

Time (s)

Speed (mi/h) and Distance (ft)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

g or Proportion Brake/Accelerator

Speed (mi/h) Distance to Stop Bar (ft) Accerator Angle (Proportion) Brake Displacement (Proportion) Deceleration (g)

Accelerator Response

Brake Response

Maximum Deceleration

1 ft = 0.305 m 

1 mi/h = 1.61 km/h 

Figure 5. Line graph. Example 1 of data from Condition 3 participants who responded by 

stopping. 

Time (s) 

Speed (mi/h) and Distance (ft) 

g or Proportion Brake/Accelerator 
20 

0

20

40

60

80

100

120

140

160

180

200

0.000

0.483

0.950

1.417

1.883

2.350

2.816

3.283

3.750

4.216

4.683

5.150

5.616

6.083

6.549

7.016

7.483

Time (s)

Speed (mi/h) and Distance (ft)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

g or Proportion Brake/Accelerator

Speed (mi/h) Distance to Stop Bar (ft) Accerator Angle (Proportion) Brake Displacement (Proportion) Deceleration (g)

Accelerator Response

Brake Response

Maximum Deceleration

1 ft = 0.305 m 

1 mi/h = 1.61 km/h 

Figure 6. Example 2 of data from Condition 3 participants who responded by stopping. 

Studies that measure deceleration from film or video usually report the deceleration rate as the 

change in velocity from when the brakes lights illuminate until the vehicle stops. This is a 

different measure than the maximum deceleration, which is reported in the previous figures. For 

comparison with studies that report deceleration rate, a comparable measure was computed for 

the Condition 3 participants who were delayed by 1 s or more. This measure covered the interval 

between when the brake response began until the participant either: (1) crossed the stop line, 

(2) came to a complete stop, or (3) ceased decelerating. The latter was used when a participant 

braked so hard that he or she was approaching a stop well short of the stop line and began 

accelerating or coasting to close that distance, as was the case for the participant whose data are 

plotted in. The 15th- and 85th-percentile deceleration rates were 0.50 g (5.73 m/s (18.79 ft/s)) and 

0.63 g (5.78 m/s (18.94 ft/s)). Gates et al. reported 50th-percentile deceleration rates for first to 

stop vehicles of 0.31 g (3.05 m/s (10 ft/s)).(5) Wortman and Matthias report 50th-percentile 

decelerations of about 0.45 g (3.5 m/s (11.6 ft/s)) for the first car to stop at signalized 

intersections in Phoenix and Tucson.(15,16) Speed (mi/h) and Distance (ft) 

g or Proportion Brake/Accelerator 

Time (s) 
21 

DISCUSSION—DRIVING SIMULATION 

The present finding that 92 percent (Condition 3, trigger distance 66 m (215 ft)) of potential 

victims could be delayed (using the Ferlis assumption that 80 percent of potential violators might 

be stopped by a warning) suggests the estimated number of collisions resulting from red light 

violations that could be prevented could rise to 99 percent. Of course a 90-percent success rate 

may be optimistic. However, if only 59 percent of potential victims responded, as was the case 

for Condition 3 with a 55-m (180-ft) trigger distance, and only 50 percent of violators could be 

stopped, then the combined crash reduction due to victim and violator responses would still be a 

substantial 80 percent. 

23 

TEST TRACK (EXPERIMENT 2) 

Experiment 2 was conducted to validate the following findings from the driving simulation: 

• About 54 percent of drivers might respond to a conspicuous infrastructure-based warning. 

• A high percentage of drivers (27 percent in the driving simulation) who are traveling at 

about 72 km/h (45 mi/h) would stop for an amber signal that comes on when they are 

55 m (180 ft) from an intersection. 

• The mean peak deceleration of 0.78 g is comparable to the decelerations that might be 

seen in a real-world warning situation. 

METHODS—TEST TRACK 

The test track experiment was conducted on the Virginia Smart Road at the Virginia Tech 

Transportation Institute. A late-model passenger car was used, and the road was closed to traffic. 

The violator warning closely resembled the mast arm warning stimulus used in the driving 

simulation. It incorporated an immediate change from green to red, alternating flashing red 

beacons on either side of the solid red lens, a changeable message sign with stop symbol, and 

strobe lights that alternated at 2 Hz. The test track warning did not include intelligent rumble 

strips because that technology does not yet exist, and it did not include in-roadway LEDs 

because of cost. 

Conditions Tested 

Only one warning condition was tested, and this condition was similar to Condition 2 in the 

driving simulation. The mast arm warning used on the test track is shown in figure 7. It varied 

from the simulator mast arm in that the solid red stop symbol was replaced with an octagonal red 

outline that enclosed the word “STOP,” and brighter strobes were suspended below the sign. The 

strobe lights within the changeable message sign that can be seen in figure 7 were disconnected 

and did not flash. 
24 

Figure 7. Photo. Warning configuration on the test track. 

Trigger Distances 

Two warning trigger distances were used for the driving simulation, one at a 32-m (105-ft) 

distance and the other at a 55-m (180-ft) distance. The 32-m (105-ft) distance was selected as a 

distance at which stopping responses would be improbable but still close enough that drivers 

might attempt to brake in response. Table 7 shows the time to the stop line from 32 m (105 ft) 

and 55 m (180 ft), given a 72 km/h (45 mi/h) approach speed. It also shows the constant 

deceleration that would be required given a 1-s delay for driver reaction time and brake system 

response lag. 

Table 7. Time to stop line when warning is triggered, and required constant deceleration to 

stop at stop line assuming a 1 s delay before the deceleration begins. 

Distance to Stop line Metric Distance Time to Stop line Required Deceleration (g)

105 ft 32 m 1.6 s 1.6 

180 ft 55 m 2.7 s 0.6 

 1 ft = 0.305 m 

Participants 

Participants were licensed drivers recruited from the Virginia Tech participant database, by 

word-of-mouth, and through advertisements in a local Blacksburg, VA, newspaper. Participants 

were randomly assigned to warning distance groups with the constraint that roughly equal 

numbers of younger and older males and females were assigned to each group. The younger 

group consisted of 32 persons, with a mean age of 40 years and a range of 20 to 64 years. The 

older group had a mean age of 71 years, with a range of 65 to 82 years. Table 8 shows the 

demographic distribution of the participants among conditions. 
25 

Table 8. Age and gender distribution of participants in the two warning distance 

conditions. 

 20–64-yearold males 20–64-yearold females 65–85-yearold males 65–85-yearold females Total 

Warning at 105 ft 8 7 7 8 30 

Warning at 180 ft 8 9 7 6 30 

Total 16 16 14 14 60 

1 ft = 0.305 m 

Protocol 

Participants were told that the experiment was an evaluation of signal timing. They were not told 

that an emergency warning would be presented or that the research concerned red light running. 

They were instructed to maintain a speed of 72 km/h (45 mi/h) except when turning around or 

responding to the traffic signal. They traveled through the intersection 12 times in each direction 

before the warning was issued on the 25th approach. 

The instructions to participants were as follows: 

In this study, we ask that you drive as you normally would and 

obey all traffic laws. Assume that the speed limit is 45 mph, and 

try to maintain that speed when not constrained by curves or traffic 

signals. The road you will be driving on is closed to other traffic. 

However, you should remain alert to hazards that might include 

other vehicles, animals, or debris on the roadway. The 

experimenter’s assistant will prompt you to “watch your speed” if, 

for no apparent reason, you exceed 50 mph or drop below 40 mph. 

That prompt is a reminder that you should maintain 45 mph when 

cruising. 

The road on which you will drive has one traffic signal. Respond 

to that signal as you normally respond to traffic signals. We are 

interested in how the timing of signal changes affects driver 

response, so it is important that you respond the way you would on 

open roadways where other vehicles might be present. 

There is a turnaround at each end of the closed road. Continue to 

use these turnarounds until the experimenter’s assistant says that 

the experiment is over. However, should you feel the need to take a 

break, particularly if you become drowsy or fatigued, inform the 

assistant that you need to take a break. You may request a break at 

any time, but it is best if you request the break immediately after 

you have passed the traffic signal (not while coming up to the 

signal). 
26 

Your safety is of highest concern. Above all drive carefully and do 

not take risks. You may discontinue participation at any time and 

without penalty should you feel that you are asked to do anything 

that is unsafe or improper. 

Do you have any questions? 

In the test track experiment, the amber was triggered at distances relatively near to the stop line 

on 20 of the first 24 approaches to the traffic signal. Each approach was considered to be a trial. 

The 4-s amber was triggered four times at each of five distances, where distance was measured 

between the front of the vehicle and the stop line. On only four test track trials did the signal 

remain green. Table 9 shows the five amber trigger distances and their metric equivalents. The 

order of trials with the various approach distance triggers was randomized separately for each 

participant, except the emergency warning, which was always on the 25th trail. Each trial 

condition except the warning was presented twice on upgrade legs and twice on downgrade legs. 

The emergency warning trial always occurred on the upgrade. 

Table 9. Amber signal trigger distances and their metric equivalents. 

U.S. Customary Metric 

105 ft 32 m 

180 ft 55 m 

215 ft 66 m 

290 ft 88 m 

365 ft 111 m 

1 ft = 0.305 m 

On the 25th trial, half the participants were warned at 55 m (180 ft) from the intersection, and 

half were warned at 32 m (105 ft). One concern for use of infrastructure to warn potential victims 

of red-light violators is that drivers who are close enough to the intersection to pass through 

before the violator arrives might be slowed by the warning and then become a victim because of 

their response to the warning. This concern has led to the suggestion that the warning be 

designed to be visible only to the drivers who need to be warned.(4) The 32-m (105-ft) warning 

distance was included to test whether or not it might be so close that no driver would respond. 

Table 7 shows the time to stop line from 32 m (105 ft) and 55 m (180 ft) given a 73-km/h 

(45-mi/h) approach speed. It also shows the constant deceleration that would be required given a 

1-s delay for driver reaction time and brake system response lag. Under the test conditions on the 

Smart Road, the test vehicle’s antilock braking system limited deceleration to about 0.89 g. With 

hard deceleration, a stop is possible in response to a warning from 55 m (180 ft), but stopping for 

a warning from 32 m (105 ft) would seem improbable. If drivers in this study brake hard for the 

warning that is triggered at 32 m (105 ft), this would indicate that the design of a violator 

warning device would need to consider which drivers can see the warning. If drivers do not 

respond to the warning that is triggered at 32 m (105 ft), this would imply that shielding the 

warning from those for whom it is not intended might not be critical. 
27 

Participants were tested individually under good driving conditions (daylight, dry pavement, 

etc.). Before the first trial, each participant drove down the Smart Road to its end (2.7 km 

(1.7 mi)). During that drive, participants were asked to accelerate and brake several times to 

familiarize themselves with the vehicle’s handling and braking characteristics. Before the first 

experimental trial began, participants were required to brake “as hard as you can” from a speed 

of 72 km/h (45 mi/h). This request was repeated until the participant decelerated to a minimum 

of 0.45 g. Two experimenters were present in the car with participant drivers. The experimenter 

in the front passenger seat served as a safety observer and was also tasked with providing 

instructions and answering questions from the participant. The other experimenter sat behind the 

participant and monitored the on-board computer. The on-board computer was used to send trial 

information to the traffic controller and to monitor sensor outputs. 

Test Road 

As configured for this test, the Virginia Smart Road had two lanes (one in each direction) and 

one signalized intersection. Participants drove a 1.6-km (1-mi) approach to the intersection 

followed by a 0.5-km (0.3-mi) leg to a high-speed turnaround and another 0.5-km (0.3-mi) 

approach. The road had a 3-percent grade. Because participants turned around at the end of each 

run, half the trials were on a 3-percent upgrade and half were on a 3-percent downgrade. The 

Virginia Smart Road intersection is pictured in figure 8. 

Figure 8. Photo. Virginia Smart Road intersection and the vehicle used for testing. 
28 

Test Vehicle 

A 2004 Chevrolet Impala equipped with a differential global position system receiver, a 

longitudinal accelerometer, and sensors for accelerator position and brake pressure was used for 

the test. The vehicle data stream was synchronized with changes in the traffic signal controller 

via a wireless 802.11 communications link. Accelerator and brake position were recorded at 

100 Hz, and all other sensor and vehicle data were recorded at 10 Hz. 

RESULTS—TEST TRACK 

Dependent variables were the same as in the simulator experiment: initial speed, delay, 

accelerator release time, and brake response time. 

Initial Speed 

Amber Signal 

Unlike the simulation study, initial speed did not vary between age groups. Overall, the mean 

initial speed was 73.7 km/h (45.8 mi/h). Females approached the intersections more slowly than 

males, F (1, 56) = 5.0, p < 0.05. The mean approach speed of females was 73.4 km/h (45.6 mi/h), 

and for males it was 74.5 km/h (46.3 mi/h). This small, but statistically reliable gender difference 

in mean approach speed, was the result of females slowing more as they got closer to the 

intersections, whereas the males generally maintained the same (higher) speed until the signal 

change was triggered. The interaction of gender with the linear trend in speed was significant, 

F (1, 56) = 7.5, p < 0.01. The gender by linear trend interaction can be seen in figure 9. Error 

bars in figure 9 represent two standard errors of the mean. On average, males accelerated slightly 

between 55 m (180 ft) and 32 m (105 ft); their speed at 32 m (105 ft) was significantly faster 

than at 55 m (180 ft), F ( 1, 29) = 5.4, p < 0.05. 
29 

42

43

44

45

46

47

48

105 180 215 290 365

Distance from Stop Line (ft)

Mean Speed 

(mi/h)

Female Male

1 mi/h = 1.61 km/h 

1 ft = 0.305 m 

Figure 9. Line graph. Mean speed at the onset of the amber signals as a function of gender 

and amber trigger distance. 

Warning Signal 

Mean speed when the warning was triggered was 72.3 km/h (44.9 mi/h) and did not vary 

significantly as a function of age, gender, or warning onset distance. Although the same gender 

by trigger distance trend appeared in the warning response initial speeds (females’ mean speed 

was less at 32 m (105 ft) than at 55 m (180 ft), whereas males’ mean speed was the same at both 

distances), in this case the interaction was not statistically reliable (p = 0.09). 

Delay with 4-s Amber 

The driver responses to the amber signals on their first 24 passes through the intersection have 

been extensively reported elsewhere. (See references 6, 7, 8, and 17.) This report is not intended 

to repeat those reports but rather is focused on the proportion of drivers delayed by 1 s or more 

by the onset of the amber signals. However, because all drivers who were delayed by 1 s or more 

also came to a full stop, the likelihood of stopping is reported. This likelihood of stopping may 

be compared to the likelihood of delay in the control condition of the driving simulation. 

No trends were identified in the likelihood of participants stopping as a function of the number of 

previous passes through the intersection. Participants stopped for 55 percent of the 4-s ambers 

that were triggered at 55 m (180 ft). Participants were more likely to stop for the 55-m (180-ft) 
30 

amber onset in the test track experiment than in the simulation, χ 2 (1) = 21.4, p < 0.01. This 

finding supports the hypothesis that 55 m (180 ft) is within the dilemma zone when travel speed 

is 72 km/h (45 mi/h). Table 10 shows the percent of trials in which participants stopped 

following onset amber signals as a function of the trigger distance. 

Table 10. The percent of trials in which participants stopped response to 4-s amber onset 

shown as a function of trigger distance. 

Distance (ft) Percent of trials in which 

driver stopped 

105 3% 

180 55% 

215 81% 

290 95% 

365 100% 

1 ft = 0.305 m 

Delay Attributable to Warning 

When the warning was triggered at 55 m (180 ft), 27 of the 30 participants (90 percent) stopped 

and thus were delayed by more than 1 s. Of the three who did not stop, one was an older female 

(age 68), one a younger male (aged 20), and one an older male (aged 73). When the warning was 

triggered at 32 m (105 ft), 29 of the 30 participants (97 percent) did not stop and were not 

delayed by 1 s or more. The participant who stopped for the warning that was triggered at 32 m 

(105 ft) was a 72-year-old female. Of participants who stopped at either warning trigger distance, 

none went more than 5.2 m (17 ft) beyond the stop line. Figure 10 shows a comparison of the 

percentage of drivers who stopped for the amber warnings that were triggered at 32 m (105 ft) 

and 55 m (180 ft), with the percentage of drivers who stopped when the warning was triggered at 

these distances. As can be seen in the figure, there was no difference between responses to amber 

onset and warning onset at 32 m (105 ft), but the warning at 55 m (180 ft) was effective. 
31 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

105 ft 180 ft

Trigger Distance

Percent of Drivers 

that Stopped

Amber

Warning

1 ft = 0.305 m 

Figure 10. Bar graph. The percent of drivers in the test track experiment who stopped in 

response to amber and warning onsets at 32 m (105 ft) and 55 m (180 ft). 

Based on the initial speed, none of the 29 drivers who received the warning at 32 m (105 ft) were 

delayed more than 312 ms. Figure 11 shows that 85 percent of the delays of those who did not 

stop were less than 150 ms. 
32 

0

10

20

30

40

50

60

70

80

90

100

-13 10 33 56 79 102 125 148 171 194 217 240 263 286 309

Delay in Reaching Intersection (ms)

Cumulative 

Percent

Figure 11. Line graph. Cumulative percent of driver delays in reaching the stop line when 

the warning was triggered at 32 m (105 ft) (n = 29). 

Accelerator Release Response Time 

Accelerator release response time was the interval between the onset of the amber signal and 

when the participant began decreasing accelerator position, with the constraint that the 

accelerator position continued to zero without reversal. If the accelerator was not depressed when 

the signal changed, which was a frequent occurrence on the downgrade, no response time was 

available. Only three accelerator releases were recorded following the onset of amber signals 

with the 32 m (105 ft) trigger distance with the mean equal to 318 ms; therefore, that distance is 

omitted from figure 12 where mean accelerator release times following amber signal onset are 

summarized. Error bars represent the one standard error of the mean. The means were computed 

without regard to participants, and the number of responses contributing to each mean varied 

between 17 and 30. Larger standard errors are the result of smaller samples. It can be seen in 

figure 12 that, at the shorter trigger distances, the accelerator release averaged about 350 ms. At 

longer distances on the upgrade, participants’ response times were longer. 
33 

0

100

200

300

400

500

600

180 215 290 365

Distance from Stop Line (ft)

Mean Accelerator 

Response Time (ms)

Downgrade Upgrade

1 ft = 0.305 m 

Figure 12. Line graph. Mean accelerator release times to amber onset are shown as a 

function of grade and trigger distance. 

Brake Application Response Time 

As for the driving simulation, brake application response time was the interval between the onset 

of the amber phase and the beginning of a brake application. Because not all participants braked 

at all distances and because participants may have been applying the brake when the amber 

began, the number of participants contributing to the brake response times shown in figure 13 

varied from 3 at 32 m (105 ft) to 57 at 88 m (290 ft). The standard error of the mean is shown for 

downgrade response times. Standard errors on the upgrade were of similar magnitude, except for 

a larger error at the shortest onset distance. Response times tended to increase with distance. Mean Accelerator Response Time (ms) 

Distance from Stop Line (ft) 
34 

0

100

200

300

400

500

600

700

800

900

105 180 215 290 365

Distance from Stop Line (ft)

Mean Brake Response 

Time (ms)

Downgrade Upgrade

1 ft = 0.305 m 

Figure 13. Line graph. Mean brake response time as a function of amber trigger distance 

and grade. 

Maximum Deceleration 

The average peak deceleration for the 27 participants that stopped for the warning at 55 m 

(180 ft) was 0.67 g. This is 0.1 g less than the maximum deceleration average for the simulation 

study. 

DISCUSSION—TEST TRACK 

The test track results confirmed the findings from the driving simulation that drivers would 

decelerate rapidly for a conspicuous and novel infrastructure based warning. The finding that 

90 percent of test track drivers brought the vehicle to a complete stop within 4.6-m (15-ft) 

downstream of the stop line when the warning was triggered at 55-m (180-ft) upstream suggests 

that the hard braking observed in the driving simulation was not an artifact of the artificial 

simulation environment. The willingness of participants in the simulation to exert near maximum 

braking demonstrates that the lack of physical deceleration forces in the simulation does not 

explain the hard braking observed in the simulation. 

When the warning was triggered at 32 m (105 ft) from the stop line on the test track, 29 of 30 

drivers did not stop or slow significantly. The one driver who did stop when warned at this 

distance was delayed by more than 1 s and thus would not have been put at risk of collision with 

a red-light violator who did not slow down. This finding suggests that it may not be necessary to 

mask warnings from the view of drivers that are too close to an intersection to respond. 

However, the possibility remains that there may be an intermediate zone where drivers are less 
35 

consistent in their stop versus go behavior. Therefore, the need to mask the warning from drivers 

for whom the warning is not intended deserves further research. 

Brake and accelerator release times on the test track were shorter than those observed in the 

simulator. The significance of this difference is discussed in the next section of this report. 

37 

GENERAL DISCUSSION—DRIVING SIMULATION AND TEST TRACK 

The purpose of the simulation and test track studies was to evaluate the feasibility of warning 

potential victims of red-light violators. The results of the simulation study suggest that more than 

half of drivers would respond appropriately to an unexpected and unfamiliar warning if that 

warning were sufficiently conspicuous and distinct from a normal phase change. Although the 

simulator results are encouraging, many questions remain. Because the decelerations observed in 

the simulation were severe, one of those questions is whether or not drivers of actual vehicles on 

a real road would be willing to decelerate as severely to such a warning. To address the question 

of how drivers would behave on an actual roadway, one of the simulation conditions was 

replicated on a closed roadway. The results of that replication validate the simulation. 

Participants on the test track braked almost as severely as those in the simulation. The slightly 

greater 0.1-g deceleration that was observed in the simulation was probably attributable to the 

test track study participants’ superior ability to modulate their approach to the stop line because 

of the better distance cues provided by the realworld environment. In the simulator, participants 

frequently stopped or approached zero speed far short of the stop line—as much as 15-m (50-ft) 

upstream of the stop line. Such behavior was not observed in the field where most participants 

modulated the deceleration to come to rest within ± 0.9 m (±3 ft) of the stop line. 

Although 90 percent of the drivers stopped for the warning on the test track and only 64 percent 

stopped in the simulator (with warnings triggered at either 55 m (180) ft or 66 (215 ft)), the 

available evidence does not indicate which of these percentages is closer to what might be 

expected in an operational environment. However, there are some clues in the data. The 

accelerator and brake response time means from the test track experiment are close to those 

Olson and Sivak reported for “anticipated” stimuli.(12) This suggests that the events that led up to 

the emergency warning led participants to anticipate a stopping response. Before the warning, 

participants had driven through the intersection 24 times and had been presented with amber 

phase changes 20 times. Thus, although it is unlikely that participants anticipated the warning, 

they may well have anticipated an amber phase change, and many had stopped for amber onsets 

at the 55-m (180-ft) and 66-m (215-ft) trigger distances. This anticipation does not invalidate the 

test track findings, but it does raise the possibility that with fewer amber signals near the 

dilemma zone boundary, the test track findings might have been more similar to the “surprise” 

reaction times reported by Olson and Sivak.(10) The simulation accelerator and brake response 

times closely approximate those reported by Olson and Sivak in their “surprise” condition. 

39 

CONCLUSIONS AND RECOMMENDATIONS FOR FURTHER RESEARCH 

The results of these experiments show that drivers are likely to respond appropriately by 

stopping or slowing for an infrastructure-based warning that they have not previously 

experienced. However, before recommending that such an infrastructure-based warning system 

be deployed, several other questions need to be answered. First among these is the effect of the 

presence of other vehicles. In both of these studies, other vehicles were absent, and now that 

there is evidence that drivers will respond to warnings in the absence of other vehicles, it is 

necessary to determine what drivers would do in a stream of vehicles approaching an 

intersection. In particular, the following questions should be considered: 

• Would drivers who know there are vehicles not too far behind them brake aggressively in 

response to a warning? 

• If a driver sees preceding vehicles not braking in response to a warning, is that driver less 

likely to brake? 

• What is the probability that warnings would precipitate rear-end collisions, and how 

severe are those collisions likely to be? 

Although any rear-end collision that might result from a warning would likely be less severe than 

a side-impact collision with a red-light violator, further development of the warn-the-victim 

concept includes a need to examine and quantify the potential for rear-end collisions that may 

result from signal violation warnings. Research to address remaining questions concerning redlight violation warnings is currently being conducted by the FHWA. 

41 

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