How to Safety Rate Red Light Cameras
Rate Stoplights with Driver Braking

Factors

For calculating driver braking (and yellow light duration) vehicle distance from the intersection when light changes green to yellow worse case (hardest, maximum, braking) is used. The worse case distance is the maximum distance from the light a vehicle can legally continue (at speed), greater distances and the vehicle would run the light.

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1.) Driver Reaction Times (t_r)
Driver reaction time includes recognizing light has changed, deciding to continue or brake, and if stopping engaging the brake (remove foot from gas pedal and apply brake). Reaction times vary greatly with situation and from person to person between about 0.7 to 3 seconds or more. Some accident reconstruction specialists use 1.5 seconds as average driver reaction brake time. However, a controlled study in 2000 (IEA2000_ABS51.pdf) found average driver reaction brake time to be 2.3 seconds.

Driver Reaction Times

0.7 seconds -- about as fast as it gets 1.0 seconds -- standard in some states 1.5 seconds -- standard in some states 2.0 seconds -- standard in some states 2.3 seconds -- average 2.5 seconds -- standard in few states 3.0 seconds -- NSC and UK Standard

Driver reaction time is the biggest unknown as it varies with person. A few states (including California) have adopted, for calculation and comparison purposes, a standard driver reaction time (responding to a stoplight) of 2.5 seconds. The United Kingdom's (UK) Highway Code and the Association of Chief Police Officers ACPO Code of Practice for Operational Use of Road Policing Enforcement Technology use 3.0 seconds for driver reaction time. The National Safety Council (NSC) recommends 3 seconds minimum spacing (3 second reaction time) between vehicles traveling in the same lane.

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2.) Yellow Light Duration (t_y)
The best way to measure yellow light duration is to video record (film is as good but harder to work with) the signal light changing from green to yellow to red. Several light cycles should be recorded to check for any differences. If possible multiple stoplights should be captured (in view).

Once recorded use video editing software to time and/or count the frames the yellow is illuminated. An alternative is to use a playback machine with the option to forward/backward one frame at a time. Count the frames and divide the total by frame rate to get time. Counting frames will yield results accurate to plus or minus the inverse of the frame rate (1 frame). Accuracy is plus or minus half of 1 frame for start count, another half frame for end count (1/frame rate).

In the U.S. most video cameras record at 15, 30 or 60 frames per second (FPS). Film cameras record at 24 or 30 FPS. In Europe most video and film cameras run at 25 frames/sec. Compressed video can run at 10 or 12 FPS. Most U.S. television broadcast are at 30 frames per second, 25 FPS in Europe.

FRAME COUNTING -- Some frames may appear to have both green and yellow stopping/starting illumination, or both yellow and red stopping/starting illumination (not unusual). The most accurate way to count yellow light frames (timing) is to use the first frame the green dims and/or yellow starts to illuminate, the last frame is when the yellow starts to dim and/or the red starts to illumine. If multiple lights captured in frame (picture), whichever light signal first dims or starts to illumine is used to mark the first frame, and whichever light first dims or starts to illumine is used to mark the last frame.

Example Stoplight Video & Data.

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3.) Speed (v)
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, 15% travel faster. 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). For comparison purposes posted speed limit is an accurate number for calculations.

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4.) Road Grade (G_r)
To compensate (account) for inclines braking (g's) is adjusted by adding (downhill) or subtracting (uphill) the road grade ratio. Road grade is the ratio of elevation change to horizontal distance, and is commonly expressed as a percentage (%). Road grade may be posted on the road, if not it can be measured with an inclinometer or other method. The road grade ratio is the tangent of the angle between level and incline (horizontal distance over elevation change).

The U.S. highway systems typically has a maximum grade of 5% city and 6% mountains. The table below list elevation change and incline angle for horizontal distance change of 300 feet (100 yards) for selected road grades.

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5.) Stop Bar to Intersection Clear Line (d_i)
When a light changes to yellow a driver must decide if the vehicle can reach the clear point (be in the intersection in many states) before the light changes to red. Intersections where the stop bar (or point) is the same as the clear line, a driver has (for a 3 second yellow) 3 seconds to reach the clear point, or stop at that (the clear) point. Stopping before the clear point (such as for a crosswalk) results in less stopping distance and harder braking.

Traffic stoplights where the stop line and intersection clear line are the same, worse case (hardest) baking occurs when a light changes to yellow and the vehicle is t_y seconds (yellow light duration) from the Stop/Clear line. Intersections with different stop and clear lines, worse case braking occurs when light changes to yellow and the vehicle is t_y seconds (yellow light time duration) from the Clear Line.

Worse Case (hardest) Braking

V_o = Speed, d_i = Distance Between Stop Bar (line) and Clear Line.

STOP BAR (Line)
Many intersections have a Stop Bar (stop line), vehicles must stop before crossing the line. If there is no stop bar vehicles must stop before the crosswalk, if no crosswalk vehicles must stop before entering the intersection. The intersection is commonly defined as the extension of cross traffic curb lines.

CLEAR LINE
Clear line (point) is the location a vehicle must be at or past when a traffic signal changes to red. The clear line varies with state. Many states use the intersection as the clear line -- vehicles cannot enter intersection when or after light changes red.

Clear Line Variations include but not limited to;

If entire vehicle (not just front bumper) must be clear of some point (intersection, etc.), deceleration will depend on vehicle length -- the longer the vehicle the greater the required deceleration. The distance (d_i) between stop line and clear point is greater for longer vehicles, increasing required deceleration (braking).

The Stop Bar / Clear Intersection Line Distance (d_i) can be measured directly on site with a hand-wheel odometer, or step it off and measure your steps later. Distance can also be estimated from aerial photographs (Google Maps etc.), most have distance scales. The State Department of Transportation (DOT) should have detailed drawings, but these could be difficult, costly and time consuming to obtain.

© Copyright 2005, 2010, 2011 Donald S. Sawicki, All Rights Reserved.