Forces: Reaction Time

The reaction time of a person is the interval between receiving a stimulus and responding to it.
Reaction times typically range from 0.2 to 0.9 seconds, depending on the type of stimulus and the individual’s alertness.
An individual’s reaction time can be influenced by factors such as age, fitness, fatigue, distraction, alcohol, drugs, or the time of day.

The reaction time of a driver plays a significant role in determining the stopping distance of a vehicle.
The stopping distance is made up of the thinking distance and the braking distance. The thinking distance is the distance travelled by the vehicle during the driver’s reaction time.
The braking distance is the distance travelled by the vehicle during its deceleration to a stop after the brakes have been applied.
Slower reaction times can lead to longer thinking distances, which in turn can contribute to a longer overall stopping distance.

One method to measure reaction time involves dropping a ruler between the outstretched fingers and thumb of the test subject. The distance the ruler falls before being caught can provide a measure of the person’s reaction time.
The reaction time can be calculated from the distance fallen using ideas from the topic of forces and motion.

In road safety, a driver’s reaction time is critical since it impacts the thinking distance and thus the total stopping distance.
Educating drivers about the factors that can impair reaction time, such as alcohol, drugs, distractions or lack of sleep, is important for promoting road safety.
Road safety rules and advisories also often take into account average reaction times to set speed limits and safe following distances between vehicles.
Vehicles are designed taking into account average human reaction times. For example, brake system response times, steering capabilities, and car control systems like ABS and ESP are all calibrated to optimise control in the hands of a human driver.