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Geometric Design: Theory and Concepts

Brake Reaction Time

The brake reaction time is the amount of time that elapses between the recognition of an object or hazard in the roadway and the application of the brakes. The length of the brake reaction time varies widely between individual drivers. An alert driver may react in less than 1 second, while other drivers may require up to 3.5 seconds. 

The brake reaction time depends on an extensive list of variables, including:

  • driver characteristics such as attitude, level of fatigue, and experience.
  • environmental conditions such as the clarity of the atmosphere and the time of day
  • the properties of the hazard or object itself, such as size, color and movement.

To make highways reasonably safe, the engineer must provide a continuous sight distance (see the stopping sight distance module) equal to or greater than the stopping sight distance. As an integral part of the stopping sight distance, a value for the brake reaction time must be assumed. Extensive research has shown that 90% of the driving population can react in 2.5 seconds or less. The brake reaction time normally used in design, therefore, is 2.5 seconds. The distance traveled during the brake reaction time can be calculated by multiplying the vehicle's initial speed by the brake reaction time.

Both the brake reaction time and the braking distance are used in the calculation of the stopping sight distance. Therefore, it is suggested that you read the braking distance module before proceeding to the stopping sight distance module.