Science & Technology in Action

3rd Edition

Science and Road Safety


Driving requires constant attention and anticipation of possible hazards. The ability to react quickly to unexpected events is an important factor in preventing road accidents.
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Lesson excerpt

Every year some hundreds of people are killed on the roads in Ireland – about one a day on average. Awareness campaigns have not produced the desired change in driver behaviour. An understanding of the factors that are common to many road accidents and an understanding of the laws of motion may help increase awareness of the dangers and encourage young people to take greater care on the road whether as pedestrians, cyclists, passengers or drivers.

Reaction time (normal reaction time)
The average time for reaction to visual stimuli (with no choices) is between 180 and 200 ms (0.18–0.20 s). In the case of auditory stimuli it is shorter (140–160 ms). When decisions are required the time required to make an appropriate response to a visual stimulus increases with age from about 370 ms, for 20–30 year olds, to about 440 ms for 60–70 year olds. 

Other factors that affect reaction time are:

  • the complexity of the situation
  • the illumination (dim situations require more time to more interpret).
  • a driver’s general level of alertness
  • normal but unexpected events (e.g. brake lights on the car ahead).
  • unusual and unexpected events (e.g. car approaching on the wrong side of the road).
  • blood alcohol level, which is a factor in about 33% of road accidents, increases reaction time and impairs judgement.

Many studies have shown that 95% of real response times of drivers are in the range 1.5 to 2.5 seconds; the faster responses were recorded when drivers were warned that there might be a hazard and were therefore in a state of readiness.

Speed and braking distance
The stopping distance for a car depends on driver reaction time, the speed of the car, the slope and condition of the road surface, the condition of the tyres and the brake system of the car. In bringing a car to rest its kinetic energy must be dissipated in some way; ideally it generates heat in the brakes. If the brakes are applied so hard that the wheels cannot turn then the car skids and frictional energy is dissipated in the tyres and the road; when this happens the car cannot be steered and the stopping distanceincreases.

True or False?

  1. Stopping distance is not the same as braking distance true
  2. Most drivers can respond to unexpected situations in less than a second. false
  3. Work equals force multiplied by distance. true
  4. Kinetic energy is proportional to speed. false
  5. Over 10,000 people are injured on Irish road each year. true
  6. Friction between car wheels and the road is essential for accelerating; braking and turning or steering true
  7. Over inflation of tyres increases the braking distance. true
  8. If the wheels lock (are unable to rotate) the braking distance is reduced. false
  9. The braking distance of a car is unaffected by rain. false
  10. Aquaplaning occurs in wet conditions and does not depend on speed. false
  11. A car travelling at 120 km/h covers 33 m in one second. true