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Braking Factors

Driver & Vehicle 1.) Driver Reaction Time (tr) 2.) Vehicle Length (L) Stop Light & Road 3.) Speed (v) 4.) Yellow Light Duration (ty) 5.) Stop Line to Threshold Line (dt) 6.) Road Grade (G)

1.) Driver Reaction Times (t_r)
Driver reaction time includes recognizing the light has changed, deciding to continue or brake, and if stopping engaging the brake (remove foot from accelerator and apply brake). Reaction times vary greatly with situation and from person to person between about 0.7 to 3 seconds (sec or s) or more. Some accident reconstruction specialists use 1.5 seconds. 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.

A few states, including California, have adopted a standard driver reaction time of 2.5 seconds. The United Kingdom's 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.

Test out your reaction time by playing the PassMeFast Emergency Stop game.

2.) Vehicle Length (L)
Vehicle length is a factor because the entire vehicle must clear of the road sensor, radar beam, or laser beam, to avoid triggering the camera.

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.

4.) Yellow Light Duration (t_y)
Yellow light times vary and are not always properly set. The best way to determine yellow light duration is to measure it. The easiest way to accurately measure yellow light duration is to video record the light, and count the 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.

Once recorded use video editing software to time the yellow light frames. Alternatively, most video playback machines/software allows the user to advance or recede one frame at a time. Count the number of frames the light is yellow and divide that number by the frame rate of the video. Counting frames will yield results accurate to plus or minus the inverse of the frame rate.

In the U.S. video cameras commonly record at 15, 30 (broadcast DTV), or 60 frames per second (FPS). Hollywood and home film cameras typically record at 24 FPS. In Europe most video cameras, film cameras, and DTV broadcast operate at 25 FPS. Compressed video can be 10 or 12 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 1. Count the frames until the yellow starts to dim and/or the red starts to illuminate, this is the last frame number. Subtract 1 from the last frame number to get the frame count (number of frames). 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 Intersection Threshold (Clear) Line (d_t)
See Red Light Timing Issues - Intersection Threshold for a detailed description.

At least one red light camera manufacturer (Redflex) uses pole mounted radars to detect violations. The threshold line is set for the locality by adjusting the radar beams and software. The threshold line is difficult or impossible to determine by site inspection.

Radar Detection

Radar sample time is 50 milliseconds (20 samples per second), and has a range of about 350 feet.

American Traffic Solutions (ATS) uses magneto resistive sensors buried in the road. The system uses 2 sensors in each lane, centered in the lane and spaced 5 feet apart. Each sensor is about 4 inches in diameter (and 2 inches deep), and is battery powered and wireless (2.400 - 2.4835 GHz / 2 mW). The intersection threshold clear line is located several feet past the last sensor and depends on timing computer settings and sensor sensitivity.

Magneto Resistive Sensor Detection

Magneto resistive sensors are commonly used to measure highway traffic speeds and vehicle length. The sensors are typically spaced 10 to 20 feet apart in this application.

Other sensors include lasers, inductive magnetic loops buried in the road, and piezo ceramic pressure sensors buried in the road.

6.) Road Grade (G_r)
Braking (g's) on inclines 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 with level, or the ratio of the elevation change distance divided by horizontal distance.

Road Grade ratio (G_r)


G_r = tan(ß) = y/x

Gr	= road grade ratio
ß	= road incline angle
y	= elevation difference
x	= horizontal distance

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.