Fri | Apr 12, 2024

Why speed matters

Published:Saturday | February 24, 2024 | 12:06 AM


There are images in the media of mangled wrecks of vehicles involved in road traffic crashes, with reports of serious injuries and fatalities. Speed is commonly highlighted as a contributing factor with the appeals to “slow down”. Why is speed considered such an important determinant of the outcome of road traffic crashes? At 50 kilometres (km) an hour your vehicle is covering distance at the rate of 14 metres or 42 feet per second. At 100 km/hour the distance covered per second is 28 metres or 84 feet.

With a reaction time of one second, it means that at 100 km/hour, you would have covered 28 metres before you apply the brake to begin the process of slowing down. This distance is longer than a cricket pitch. The vehicle, dependent on the efficiency of the brakes, the quality of the tyres and the nature of the road surface may come to a stop at some variable distance. If reaction time is two seconds, then the distance doubles and so on. Your reaction time and the braking effect might not translate into a safe stopping distance.

It is important to note that three collisions occur in any given crash. The first collision is between the vehicle and another external object e.g. another vehicle, wall or pedestrian. The second collision is between the occupant and the vehicle, e.g., steering wheel, dashboard or windshield. The third and final collision occurs once the body comes to a halt. It involves organs smashing against each other or the inside of one’s body. Softer organs often move forward until halted, resulting in bruising or tearing. Other organs like the heart or the brain can also undergo so much trauma that they either rupture or stop functioning.

A factor which significantly predicts injury, is energy transfer. This is the force which is applied to the body because of the impact. Energy transfer is related to speed. The higher the speed, the higher the energy transfer and the greater the injury. In fact, the energy in a crash is proportional to the square of the speed. If you travel at 100km/hour versus 50km/hour you’re increasing the amount of energy that has to be dissipated (back on to the car and to you) by a factor of four. This is the rationale behind crumple zones and the use of seatbelts and airbags which are meant to reduce energy transfer to the body during impact.

Speed in the road environment reduces your ability to control the vehicle, as well as the ability to control the outcome upon applying the brakes, or the outcome on impact. Is the risk of the unknown outcome to yourself, your passengers, and fellow road users worth the speed?