Perception – Reaction – Braking

Hugh Borbidge, BSME, Director of Engineering Animations

 

 

We, as vehicle operators, avoid collisions every day. We stop for red lights, brake when a car cuts us off, and slow down if there are children playing close to the roadway. In these avoidance situations, there are three components: Perception, reaction and braking.

Perception is when we first notice a hazard or the need to react to something. Reaction is the time taken to respond to what was perceived. A typical daytime perception/reaction time for a vehicle operator is about 1.5 seconds, meaning that, when you notice a hazard you keep traveling at the same speed for 1.5 seconds before you start applying the brakes. Braking is the minimum time it takes to bring the vehicle to a stop. Every bit of time in this series of events equates to a certain distance traveled. For example:

  • A vehicle traveling at 25mph travels 55 feet during its perception/reaction time and takes 30 feet to stop for a total of 85 feet in 3.13 seconds.
  • A vehicle traveling at 45mph travels 99 feet during its perception/reaction time and takes 97 feet to stop for a total of 196 feet in 4.43 seconds.
  • A vehicle traveling at 65mph travels 143 feet during its perception/reaction time and takes 202 feet to stop for a total of 345 feet in 5.73 seconds.

In summary, the faster you are traveling, the more time and distance is required to bring your vehicle to a stop.

Hugh Borbidge, BSME, Director of Engineering Animations with DJS Associates, Inc., can be reached via email at experts@forensicDJS.com or via phone at 215-659-2010.

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