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Math Page

1.  Stopping Distance Chart

This is an exact copy of the CHP's chart, which was available on their website until May 2004.  CHP public information staff said that they removed it from the site because "It didn't take into account ABS brakes."  So far, they have not offered a replacement chart.

This chart is based upon very hard "panic" braking at 0.7 G, which is a decrease in your speed of 22 feet per second, each second (in engineering terms, 22 feet per second squared), while a comfortable stop is one with a deceleration rate less than half that.  From FAQ #6 on the Links Page:  "Olson and Rothery reported in 1972 that their research showed that drivers were "virtually" certain to stop if their required deceleration rate was less than 8 feet per second squared and virtually certain to continue if the deceleration rate required was in excess of 12 feet per second squared" [Determining Vehicle Change Intervals - A Proposed Recommended Practice," ITE, 1985]

 Stopping Distances for Passenger Vehicles (chart)

 Speed (Miles Per Hour) Speed (Feet Per Second) Perception + Reaction Distance "Brake Lag" Distance Effective Braking Distance Total Stopping Distance 15 22.0 33 0 11 44 20 29.3 44 0 19 63 25 36.6 55 0 30 85 30 44.0 66 0 43 109 40 58.6 88 0 76 164 50 73.3 110 0 119 229 55 80.6 121 0 144 265 60 88.0 132 0 171 303

1. Perception + Reaction Distance = Feet Traveled in 1.5 Seconds. The average driver's perception time is .75 seconds and the average reaction time is also .75 seconds.
2. "Brake Lag" Distance = Brake lag in a passenger vehicle is approximately .05 of a second. This figure is generally not taken into consideration in determining stopping distances.
3. Effective Braking Distance = Feet Traveled after brakes make contact with drums / rotors with good braking efficiency on good dry level pavement. Coefficient of Friction = .70 . Decelerating Rate = 22.5 feet per second average.
4. Total Stopping Distance = Perception + Reaction + Brake Lag + Effective Braking Distance.
5.

2.  How Far Back Were You?

The top photo on your ticket shows your car very close to the limit line.  If you want to know where your car was at the instant the electricity was first applied to the red signal lamp, multiply your Late Time times your speed (in feet per second).

Most tickets show your speed in miles per hour.  To convert to feet per second, use the CHP chart, above, or multiply by 1.5.

3.  Interpreting the Speed Shown on the Ticket

Most tickets tell you what speed you were going.  It can usually be found in the data box or strip at the top of the first photo they took.

Some ticket formats also print a speed figure on the second photo.  If you are trying to make some time/distance calculations and can't get them to come out right, it could be because the speed printed on the second picture is usually just a repeat of the speed reading made by the loops your car passed over at the place where the first photo was taken.  The speed shown on the second photo will usually not be a new reading.

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