Police Radar Handbook

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Red Light Camera Timing Issues

Worse Case Distance and Braking
when Stop Light changes Yellow

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

Worse Case Location | Braking Rate | 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 not adjusted.

Drivers that just happen to be in the worse case location when a green light changes to yellow will experience worse case hardest braking or run the light. Worse case distance and braking can be accurately calculated by knowing yellow light duration, speed, distance between stop line and violation point (varies with local laws), and driver reaction time. Road grade is an additional factor for hills.

A vehicle cannot enter or be on the intersection threshold line when a stoplight changes red. The intersection threshold line varies with local laws and ranges from before the stop line to the cross traffic curb line to past the intersection. The International Association of Chiefs of Police Performance Specifications (RLCS_Module_11-29-07_d2.doc) for Red Light cameras defines the intersection threshold and a violation.

Intersection Threshold - A line, real or imaginary, that defines the start of the controlled intersection. The intersection threshold is perpendicular to traffic flow, located on the lanes facing the traffic signal, and its position in those lanes is defined by local law.

Red Phase Violation - An incident at a controlled intersection whereby any part of a vehicle extends past the intersection threshold while the traffic signal is displaying a red phase to the operator of such vehicle.

The Manual on Uniform Traffic Control Devices defines an intersection as the extension of the cross traffic curb line.

Worse case braking occurs when a light changes to yellow and a vehicle is the yellow light time from the camera trigger line. The rear of the vehicle must be past the intersection threshold by the vehicle length, if not the camera triggers and registers a violation. The camera trigger line varies with vehicle length, and the intersection threshold line varies with local law.

Worse Case Scenario

intersection Threshold
Braking depends on camera trigger line which depends on vehicle length and intersection threshold to stop line distance. The intersection threshold line is set at, depending on local government, before, on, or past the stop line. For calculations the distance d_t is positive (+) when the threshold line is after (past) the stop line, and negative when in front of (before) the stop line.

Redflex Traffic Systems Uses radar and can adjust the intersection threshold to before, on, or after the stop line.

American Traffic Solutions (ATS) Uses road sensors placed a few feet in front of (before) the stop line.

When d_t is negative and the distance between trigger line and stop line is greater than vehicle length the vehicle front triggers the sensor. This takes the vehicle length (L) out of the stopping distance equation (if d_t > L, then L = 0).

Camera Trigger Time Delay
Some red light cameras are programmed to delay the camera trigger. A trigger delay greater than or equal to the time it takes a vehicle to cross over the intersection threshold changes the detection point from vehicle rear to front. This takes the vehicle length out of the stopping distance equation (if vt_d >= L, then L = 0). See Camera Trigger Time Delay (t_d).

Worse Case Distance General Formula

d_s = react + stopping distance
d_t = stop line to intersection threshold line
L = vehicle length
t_y = yellow light duration
t_d = camera time delay
v = vehicle speed

Worse case braking rate calculations require driver reaction time and road grade.

Braking rate depends on worse case distance factors plus road grade and driver reaction time. Different states use different driver reaction times for braking calculations, from as short as 1.0 up to 2.5 seconds. Overall average reaction time for all driver types is 2.3 seconds.

Braking rate is usually measured in "g's" - acceleration compared to acceleration due to gravity (g = 32.174 ft/s²). The "g" scale accounts for stoplights on hills, it's easier to brake going uphill and harder going downhill compared to a level road.

Braking g's (B_g)
Mathematical Model

TIME DELAY: if vtd >= L vehicle front is trigger and L = 0. Exceptions for Intersection Threshold Line before Stop Line dt is negative (-dt). If |dt| > L vehicle front is trigger and L = 0.

Online Braking Calculator

Braking Rate g's mph/sec* BRAKING 0.35 8 Safe Braking 0.45 10 Maximum Average Driver 0.65 14 Maximum Skilled Driver 0.75 17 Vehicle Maximum 1.00 22 Vehicle Max on Record * Flat and Level Road

Convert Acceleration Unit Dimensions g's ft/s*s mph/sec g's, ft/s*s, mph/s. Acceleration in g's, ft/s*s and mph/sec

Mathematical Models Acceleration Parameters Typical red light Camera Intersection

Yellow Light Duration Calcualtor Acceleration Table and Converter -- English and Metric units

Red Light Camera Concerns

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 action (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, -- 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.