Posted By Gordon Thelwell
is this more useful?
Impact on a pedestrian
Because the pedestrian, "Sam", is so much lighter than the car, he has little effect upon its speed. The car, however, very rapidly increases Sam's speed from zero to the impact speed of the vehicle. The time taken for this is about the time it takes for the car to travel a distance equal to Sam's thickness – about 20 centimetres. The impact speed of Car 1 in our example is about 8.1 metres per second, so the impact lasts only about 0.025 seconds. Sam must be accelerated at a rate of about 320 metres per second per second during this short time. If Sam weighs 50 kilograms, then the force required is the product of his mass and his acceleration – about 16,000 newtons or about 1.6 tonnes weight.
Since the impact force on Sam depends on the impact speed divided by the impact time, it increases as the square of the impact speed. The impact speed, as we have seen above, increases rapidly as the travel speed increases, because the brakes are unable to bring the car to a stop in time.
Once a pedestrian has been hit by a car, the probability of serious injury or death depends strongly on the impact speed. Reducing the impact speed from 60 to 50 kilometres/hour almost halves the likelihood of death, but has relatively little influence on the likelihood of injury, which remains close to 100 per cent. Reducing the speed to 40 kilometres/hour, as in school zones, reduces the likelihood of death by a factor of 4 compared with 60 kilometres/hour, and of course the likelihood of an impact is also dramatically reduced.
Modern cars with low streamlined bonnets are more pedestrian-friendly than upright designs, such as those found in 4-wheel drive vehicles, since the pedestrian is thrown upwards towards the windscreen with a corresponding slowing of the impact. Cars with bullbars are particularly unfriendly to pedestrians and to other vehicles, since they are designed to protect their own occupants with little regard to others