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Red Light Camera
Safety Issues

Determine if a Red Light Camera
is for Safety or a Dangerous Tax

Braking Factors | Worse Case Distance | Braking Action | Concerns

Drivers at the wrong distance from the Stop Line when a light changes to yellow will have the shortest distance to brake. The difference between a safe and unsafe braking distance mainly depends on yellow duration and driver reaction time. Driver reaction times vary from 1 to 3 seconds, but many lights assume a 1 second reaction time, an old and outdated standard (see below). In the past some communities set the yellow cycle short to get more violations and money.

Braking Factors

Braking force depends on the stop light parameters, road grade, and driver and vehicle. Vehicle length matters because the entire vehicle must be in the intersection, past the violation line, when the light changes red.

1.) Driver Reaction Times

Driver reaction time varies with individual from maybe 1 second up to 3 seconds. A half second can make a huge difference in braking action. A controlled study in 2000 (IEA2000_ABS51.pdf) found average driver reaction brake time to be 2.3 seconds. The study included all driver types. Test were conducted on a controlled track and in a driving simulator.

Test out your reaction time on the website PassMeFast Emergency Stop game.

2.) Vehicle Length
Vehicle length is a factor because the entire vehicle must clear of the Violation Line (see below) . The longer the vehicle the shorter the time the vehicle has to clear the violation line.

3.) Speed
Standard practice is suppose to be speed limits are set to the 85 percentile speed. The 85 percentile speed is the speed 85% of traffic travels at or below. A traffic study is suppose to be conducted over a relatively long period to determine the 85 percentile speed empirically. Some places set the limit 8 to 12 mph below the 85 percentile speed, resulting in more speeders and red light runners.

4.) Yellow Light Duration
Yellow light times are not always properly set. The easiest way to accurately determine yellow light duration is to video record the light, and count the number of frames the light is yellow. Several light cycles should be recorded to check for any differences. If possible multiple stoplights should be in the viewing frame.

Count the number of yellow frames and divide that number by the video frame rate (fps - frames per second). Results are accurate to plus or minus the inverse of the frame rate. In the US 30 fps is common. Europe uses 25 fps. Most Hollywood movies use 24 fps.

FRAME COUNTING -- It may not be exactly clear when the yellow light starts and ends, such as multiple light colors appearing in a frame. The most accurate way to count frames is to use the first frame the green dims and/or the yellow starts to illuminate as frame number "0". Count the frames until the yellow starts to dim and/or the red starts to illuminate, this is the frame count. If multiple poles or hanging lights are captured in a frame, whichever light first dims or starts to illumine is used as a frame number start or end.

5.) Stop Line to Violation Line Distance (d_i)
The stop line to violation line distance is a factor that effects braking.

• Stop Line
  The line location varies with intersection and is usually based on US Department of Transportation standards.
• Violation Line
  The line location is set by local law and defines the start of the intersection. It's location varies from the same as the stop line (d_i =0) to even with the cross tarffic curb.

VIOLATION
The entire vehicle must be passed the violation line when the light changes red. The International Association of Chiefs of Police (IAPC) define a red light violation.

Performance Specifications for Red Light Cameras (.pdf)
1.4.16 -- Red Phase Violation
An incident at a controlled intersection whereby any part of a vehicle extends past the intersection threshold (Violation Line) while the traffic signal is displaying a red phase to the operator of such vehicle.

See Violation Detection Methods.

6.) Road Grade
Road grade is the ratio of elevation change to horizontal distance, and is commonly expressed as a percentage (%). The U.S. highway systems typically has a maximum grade of 5% city and 6% mountains.

Road Grade (G)

G = tan(ß) = y/x

ß = Inclination Angle G = Road Grade +G uphill, -G downhill

X = Horizontal Distance y = Elevation Difference

CALCULATE ROAD GRADE

Calculate Road Grade from Inclination Angle
Incline Angle:		degrees (°)
Road Grade Ratio Percent Calculate or Re-calculate

Calculate Road Grade from X and Y
Horizontal Distance (x):		units
Elevation Change (y):		units
Units can be any length measurement, feet, meters, etc
Road Grade Ratio Percent Calculate or Re-calculate

Worse Case Location when Light changes Yellow

Worse Case Distance
from Stop Line (d_s)

ds = v ty - di - L

ds	=	Distance to Stop Line (ft)
v	=	Speed Limit (ft/sec)
ty	=	Yellow Light Duration (sec)
di	=	Stop Line to Violation Line (ft)
L	=	Vehicle Length (ft)

Acceleration or deceleration is the change in speed over time. Road grade makes it harder to decelerate downhill, easier for uphill. Effective deceleration uses road grade to account for hills.

Braking Factors
- Speed - Road Grade - Vehicle Length		- Yellow duration - Driver Reaction Time - Stop to Violation Line Distance

Deceleration is measured in ft/sec², mph/sec, or
g's referenced to the Earth's Gravity Constant
32.174 ft/sec².

Driver Braking Limits
g's	mph/sec	ft/sec2	BRAKING
0.30 0.35	6.6 7.7	9.7 11.3	Safe Braking
0.47	10.3	15.1	Average Driver Maximum
0.62	13.6	19.9	Reasonably Skilled Driver Maximum
0.65	14.3	21.0	Skilled Driver Maximum
0.70 1.0	15.4 21.9	22.6 32.1	Vehicle Maximum

See Braking Model and Formula.

CALCULATE WORSE CASE BRAKING
Driver and Vehicle
1.)	Driver Reaction Time:		seconds
2.)	Vehicle Length:		feet
Stop Light and Road
3.)	Speed Limit:		mph
4.)	Yellow Duration:		seconds
5.)	Stop to Violation Line:		feet
6.)	Road Grade: Negative for Downhill		ratio %
Braking Rate g's ft / sec2 mph / sec Description Calculate or Re-calculate

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Red Light Camera Concerns

Many communities use red light cameras as a revenue source, then claim it is for safety. Some places go so far as to shorten the yellow duration forcing more drivers to run the light. To make matters worse many states use an unrealistic (fast) driver reaction time to determine yellow light duration. Another reason for short yellow time is the intersection was changed, lanes added or speed limit changed, but the intersection stop light was never adjusted.

Cameras do not improve safety or fix problematic intersections. Intersections can usually be fixed by increasing yellow light duration. Longer yellow light duration lowers braking rate (safer stops). Cameras have been known to cause collisions. Some drivers aware of a camera brake too soon and get rear-ended. The person the vehicle is registered to gets ticketed. Person vehicle registered to may not have been driving. Presumption the owner is guilty (until proven innocence) Evidence against vehicle, not driver (no direct evidence against owner). The process is automatic. A police officer does not have to see alleged violation, You cannot cross examine electronic circuits (loss of right to confront witness). Camera's real purpose is to collect lots of $money$. Sometimes the yellow duration is decreased (more tickets and more dangerous). Up to half of fine can go to a 3rd party (private company that maintains system). Some contacts forbid increasing (making safer) yellow duration.

Any measuring device, especially automatic devices (speed and red light cameras), should have established tests (calibrations and/or laboratory test) scheduled on a regular basis. Test records should be maintained and available. Any device not tested and maintained properly has questionable accuracy. Also the chain of control of evidence (photos and data) should have checks and balances with accountability and be well established and followed.