Home


stop light stop light

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.

Worse Case Location
when Light changes Yellow

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
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 dt is positive (+) when the threshold line is after (past) the stop line, and negative when in front of (before) the stop line.
Stop to Photo Line distance

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

American Traffic Solutions (ATS)
Uses road sensors placed a few feet
in front of (before) the stop line.
When dt 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 dt > 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 vtd >= L, then L = 0). See Camera Trigger Time Delay (td).

Worse Case Distance General Formula
Distance from Stop Line
ds = v (ty + td) - (dt + L)
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.
ds = react + stopping distance
dt = stop line to intersection threshold line
L = vehicle length
ty = yellow light duration
td = camera time delay
v = vehicle speed
Calculate Worse Case Distance
v = Speed Limit: mph
L = Vehicle Length: feet
ty = Yellow Duration: seconds
dt = Stop Line to Intersection:
(-) if Intersection Threshold before Stop
feet
td = Trigger Delay: seconds

Worse Case Distance


Approximate Vehicle Lengths
13 feet - small car
15 feet - midsize car
16 feet - large car, small pickup or van
18 feet - pickup truck or van
20 feet - large pickup truck
Worse case braking rate calculations require driver reaction time and road grade.

Braking Rate
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 Factors
Driver and Vehicle
tr = Driver Reaction Time
L = Vehicle Length
Stop Light and Road Data
v = Speed Limit
ty = Yellow Light Duration
Gr = Road Grade
dt = Stop Line to Intersection Threshold Line
td = Photo Time Delay
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 (Bg)
Mathematical Model

Braking g's
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/s.



Red Light Camera Concerns

sign
  • 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.

CopRadar.com
Red Light Camera Timing Issues
Home | Top
© Copyright 2005 - 2013 Donald S. Sawicki, All Rights Reserved.