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RedLight Cameras


... Technical State University, Greensboro, NC, July 2004. ... Burkey and Obeng, Greensboro, NC, July 2004. Large data base yields good statistical accuracy: ... – PowerPoint PPT presentation

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Title: RedLight Cameras

Red-Light Cameras
  • The Good
  • The Bad
  • The Uncertain

Dale Gedcke, B.Eng., M.Sc., Ph.D. Marketing
Technical Consultant Oak Ridge, TN
OR RLC Presentation 5-18-08.ppt
How RLCs Work
Traffic Light
Crossing sensor on red captures 6 sec of video
If you cross the sensor on red, about 2 weeks
later a citation like this arrives in the
mail for the owner of the vehicle.
The black bars include the time, place, speed
limit, your speed, and amount of time the light
was red when your crossed the sensor.
Where to view the video on-line.
Red-Light Violation Response Options
  • View video on-line, then chose one of 3 options
  • 1) Pay 50
  • No points on license
  • No notification to insurance company
  • No notification to TN Dept. of Safety
  • 2) Name the actual driver
  • Affidavit
  • Vehicle owner is liable if actual driver fails to
  • No notification to Insurance Company, nor TN
    Dept. of Safety
  • 3) Contest the ticket in court. If you lose
  • Additional 8 scheduling fee, plus 60 court fee
  • Judgement becomes a public record available to
    data mining companies

Knoxville, TN / Redflex Citations in 2007
  • Revenue Sharing Formula
  • lt 4500 per camera per month 15 to City, 85
    to Redflex
  • gt 4500 per camera per month 50 to City, 50
    to Redflex
  • 15 Intersections with cameras
  • 60,299 Red-light violations
  • 955,014 to City revenue
  • 1,644,719 to Redflex revenue
  • Net 43.11 per initial citation. (Compare to
    50 ticket.)

RLC Safety Premise
Installing Red-Light Cameras should improve
safety because
  • Drivers running a red light risk a collision with
    cross-traffic operating on green
  • Red-Light Cameras catch ALL red-light violations
  • Drivers soon learn to stop on red to avoid a

Why Do Drivers Run Red Lights?
  • In a rush. Tried to beat the red. (Sensitive to
  • Misjudged time versus distance (Minor sensitivity
    to RLC)
  • Thought there was time to make it on yellow
  • Distracted when it changed to yellow
  • Did not see the signal (Not sensitive to RLC)
  • Visibility problem
  • Could not stop in time (Not sensitive to RLC)
  • Dilemma Zone (yellow too short for speed limit)
  • CONCLUSION RLC will not suppress all red-light

Proving/Disproving the RLC Safety Premise
  • Simple analyses are almost always misleading
  • Requires a comprehensive and sound statistical
    analysis because
  • Many confounding variables (traffic rate, traffic
    patterns, weather, intersection characteristics,
    speed, time of day, day of week, changes in
    vehicle safety features, driver characteristics,
    truck traffic, yellow duration, all-red duration,
  • Must correct for changes in traffic volume
  • Must compare the before and after accident rates
    to similar intersections without RLCs (to
    determine what changes were due to the RLCs)
  • Important to test for spill-over effects at
    nearby intersections without RLCs
  • Paucity of data and poor quality of data
  • Dealing with low numbers of random accidents
    Statistical uncertainty is large.
  • To detect a 10 difference with 95 confidence
    requires gt800 accidents Before/After RLC
    installation and gt1600 Before/After at non-RLC
    control sites.
  • Must use tests for statistical significance of
  • t-test, Chi-squared test, F-test, variance
    analysis, regression analysis, empirical Bayes
  • If an analysis does not use any of these tests,
    its conclusions are worthless.

Several Sound Statistical Studies are Available
  • Barbara Langland-Orban et al., Red Light Running
    Cameras Would Crashes, Injuries and Automobile
    Insurance Rates Increase If They Are Used in
    Florida? Florida Public Health Review, 2008
    51-7, and 5 47-52.
  • Nattaporn Yaungyai, Evaluation Update of Red
    Light Camera Programming in Fairfax County,
    Virginia Masters Thesis, Virginia Polytechnic
    Institute and State University, April 2004.
  • Mark Burkey and Kofi Obeng, A Detailed
    Investigation of Crash Risk Reduction Resulting
    from Red Light Cameras in Small Urban Areas,
    North Carolina Agricultural Technical State
    University, Greensboro, NC, July 2004.
  • Nicholas J. Garber et al., The Impact of Red
    Light Cameras (Photo-Red Enforcement) on Crashes
    in Virginia, Virginia Transportation Research
    Council, Charlottesville, VA, June 2007.

Barbara Langland-Orban et al., Florida, 2008
  • Excellent review and critique of all statistical
    studies performed on RLC installations to date.
  • Easy to read for the non-statistician.
  • A good place to start!

Nattaporn Yaungyai, Thesis, Virginia Polytechnic
Institute and State University, April 2004.
  • Excellent introduction to the RLC technology,
    history, laws and statistical testing process
  • Summarizes laws in all USA States extant up to
    April 2004.
  • Summarizes experience in several countries
  • Reviews public opinion surveys on RLCs in various
  • Analyzes RLC impact in Fairfax County, VA
  • 13 camera intersections 12 to 82 accidents per
    year per intersection category (RLC, comparison,
    spill-over) only 2 comparison intersections
    4453 to 887 total RLC violations per month
  • Inadequate number of accidents and too few
    comparison intersections for determining
    significant accident trends.
  • Enough RLC violations to determine RLC violation
  • RLCs reduced red-light violations as much as 58
    in 22nd - 27th month after installation
  • Lengthening yellow light duration reduced
    red-light violations as much as 70
  • Statistically non-significant reduction in
    accident rate.

Burkey and Obeng, Greensboro, NC, July 2004.
  • Large data base yields good statistical accuracy
  • 303 intersections
  • 18 RLC sites 840 accidents before 777
    accidents after RLC installation
  • 285 non-RLC sites 4827 accidents before, 4211
  • Analyzed correlation with weather
  • RLC installation associated with a statistically
    significant 40 increase in accident rates
    compared to non-RLC intersections.
  • No change in angle accident rates, and large
    increases in rear-end crash rates and other types
    of crashes relative to non-RCL intersections
  • Statistically non-significant increase in fatal
    accident rate
  • Decrease in left-turn accident rate with cross
    traffic at RLC sites
  • Longer yellow light duration decreases accident

Garber et al., VA Trans. Res. Council, 2007
  • Large data set yields good statistical accuracy
    in the aggregate
  • More than 3500 crashes over 7 years
  • 28 RLC intersections and 44 non-RCL intersections
  • 6 jurisdictions (VA) Alexandria, Arlington,
    Fairfax City, Fairfax County, Falls Church
  • Analyzes aggregate versus individual
    jurisdictions and intersection types for
    differing trends. (Statistical accuracy suffers
    when subdivided by jurisdiction or intersection.)
  • Offers guide for selecting successful
    accident-reduction RLC sites
  • Attempts cost vs. benefit analysis Inconclusive
    /- result
  • Aggregate total accident rates increased 29 with
  • Aggregate Rear-end crash rates increased 42 with
  • Aggregate red-light running crash rates decreased
    a statistically non-significant 8 with RLC
  • 4 of RLC intersections experienced increased
    angle crash rates
  • 2 of RLC intersections showed decreased rear-end
    collision rates.

Dilemma Zone
You can get trapped by a dilemma zone, if the
yellow duration is too short for the speed limit.
Traffic Light
Dilemma Zone If light turns yellow, it is
impossible to enter the intersection before red,
and impossible to stop safely before the
Dilemma Zone
Approach speed v 40 mph 58.7 ft/s Yellow
light duration tY 4.0 seconds Decision time
tD 1.00 seconds Reaction time tR 0.50
seconds Maximum safe deceleration rate a 10
ft/s/s Percent Grade (ve uphill, -ve downhill)
G 0 Maximum distance to make it on yellow
Minimum distance to stop safely (avoiding
rear-end collision)
Dilemma Zone exists from 235 to 260 ft from
intersection. If the light turns yellow when you
are in this zone you can neither continue
safely nor stop safely.
Minimum Yellow to Eliminate Dilemma Zone
From 2003 TN Traffic Design Manual, recommended
yellow light durations for level intersections
are Approach Speed (mph) 25 30 35 40
45 50 55 60 65 Yellow Duration
(sec.) 4.0 4.0 4.0 4.5 4.5 5.0
5.0 5.5 6.0 NOTE Descending grades require
longer delays. Using values from the previous
slide, the minimum yellow duration at 40 mph is
computed as
For more realistic 85th percentile reaction times
(tD tR), see 1) APPENDIX B and 2) Thomas J.
Triggs and Walter G. Harris, Reaction Time of
Drivers to Road Stimuli, Human Factors Report No.
HFR-12, ISBN 0 86746 147 0. Human Factors Group,
Department of Psychology, Monash University,
Victoria 3800, Australia, June 1982.
  • The most important and most effective safety
    solution Increase yellow duration according to
    approach speed and grade to eliminate unsafe
    Dilemma Zones.
  • Statistically sound studies show that Red-Light
    Cameras are NOT a reliably effective solution for
    improving safety.
  • The RLC effect on angle collision rates varies
    from positive to negative
  • RLCs usually significantly increase the rate of
    rear-end collisions
  • RLCs tend to increase total accident rates
  • However, RLC citation rates do decline in the
    first two years of operation
  • Red-Light Cameras generate addictively large
  • Conflict of Interest Safety can easily become
    less important than revenue
  • Analyze the operating costs carefully. Some
    municipalities have lost money because of
    increased direct labor, overhead, and court
    costs. Some have discontinued RLCs due to
    declining revenue.
  • See Appendices A through D for more details.

Appendix A Useful References
  • US DOT Federal Highway Administration Manual on
    Uniform Traffic Control Devices (MUTCD)
  • Tennessee MUTCD Traffic Design Manual
  • Making Intersections Safer A Toolbox of
    Engineering Countermeasures to Reduce Red Light
    Running, FHWA and Institute of Transportation
    Engineers, Pub. No. IR-115, ISBN 0-935403-76-0
    (2003) http//
    rlr_report/index.htm (published before the 2004
    and 2007 studies on RLCs)
  • Dr. Peter T. Martin, Vikram C. Kalyani and
    Aleksander Stefanovic, Evaluation of Advanced
    Warning Signals on High Speed Signalized
    Intersections, Univ. of Utah, Nov. 2003.
    5/index.php (Dilemma Zones)
  • Philip R. Bevington and D. Keith Robinson, Data
    Reduction and Error Analysis for the Physical
    Sciences, WCB/McGraw-Hill, ISBN 0-07-911243-9,
    1992. (Excellent introductory book on
    statistical analysis)

Appendix B Driver Reaction TimesDependent on
several factors
  • Distractions
  • Anticipation of, or focus on a particular warning
    or hazard
  • Difficulty of interpreting the significance of
    the hazard or warning
  • Difficulty of figuring out the best solution
  • Physical motion required to implement the
  • Mechanical delay in vehicle actuation
  • What is the Right Reaction Time?
  • MUTCD documents use a 1-second reaction time with
    a yellow light.
  • Dilemma Zone example used 1.5 seconds.
  • Studies showed a longer yellow reduces accidents
    and red-light violations.

Appendix B (contd.) Driver Reaction Times.1.5
sec. is a reasonable minimum
Traffic Engineers typically use the 85th
Percentile for design i.e., 85 of drivers
have a lower reaction time.
From Thomas J. Triggs and Walter G. Harris,
Reaction Time of Drivers to Road Stimuli, Human
Factors Report No. HFR-12, ISBN 0 86746 147 0.
Human Factors Group, Department of Psychology,
Monash University, Victoria 3800, Australia, June
Appendix C Required Yellow Duration and Green
Delayto Eliminate Dilemma Zone and Clear the
Green Delay is also called the All-Red phase
(red for all directions).
Appendix D Poisson Statistics.Why the Sample
Size Must Be Large
  • Accidents have a low probability
  • typically lt 50 per 1 million cars thru the
  • intersection.
  • Poisson Statistics apply the probability
  • of observing N accidents in a time period t is

m the mean or average of the distribution
A measure of the width or dispersion of the
distribution about the mean is the standard
deviation, s
Appendix D (contd.) Poisson Statistics.Why
the Sample Size Must Be Large
For a single sample of N accidents observed in a
time t, the best estimate of the mean of the
underlying probability distribution is
The best estimate of the standard deviation of
the underlying probability distribution is
The dispersion, expressed as a percent of the
mean value is
s represents the percent uncertainty in
estimating the true, mean number of accidents
from a single measurement, N.
Appendix D (contd.) Poisson Statistics.Why
the Sample Size Must Be Large
Percent Uncertainty in the Number of Accidents N
1 10 100 1,000 10,000 100,000 1,000,000 s
100 32 10 3.2 1 0.32
  • To detect changes of the order of a few percent,
    one must have
  • a sample of more than 10,000 accidents.
  • When comparing accident rates at RLC
    intersections to similar,
  • non-RLC intersections, one needs more than
    twice as many non-RLC
  • intersections to keep from degrading the
    statistical uncertainty.