Crashes and SpeedThe Formula is Deadly

In theory, speed plays a role in every road crash: if everybody were to stand still, there would be no traffic. However, it is very difficult to determine the number of crashes in which too fast a speed was the

main cause. In addition to speed, there often are various other factors involved that play a role in a crash occurring.

No Dah!

• Speed is also dangerous if it is higher than the circumstances at that moment allow (e.g. because of rain, fog or large traffic volume). In general, this inappropriate speed in particular is difficult to determine objectively. Therefore the police rarely register speed as the crash cause. It is generally assumed that about one third of fatal crashes are (partly) caused by speeding or by inappropriate speed

• The relation between speed and safety rests on two pillars. The first pillar is the relation between collision speed and the severity of a crash; the second pillar is the relation between speed and the risk of a crash. The higher the collision speed, the more serious the consequences in terms of injury and material damage. This is a law of physics that involves the quantity of kinetic energy that is converted in an instant into e.g. heat and matter distortion. In addition, the human body is physically very vulnerable in comparison with the enormous forces released in a collision. During the past decades, vehicles have become ever better equipped (with crush areas, airbags and seatbelts) to absorb the energy released in a crash, thus protecting the occupants. However, the collision speed still is very important for the crash outcome.

Stopping distance• A critical factor in the relationship between speed and crashes is

stopping distance. There are two components to stopping distance:• The distance travelled by the vehicle during the time it takes for the

driver to react; and• The distance travelled once the brakes have been applied.

The impact of speeding on crash risk• The risk of a casualty crash approximately doubles with each 5km/h

increase in speed on a 60km/h speed limited road, or with each 10km/h increase in speed on 110km/h roads.It is illegal to drive at any speed above the speed limit.

• Vehicle travel speeds affect both the risk of crash involvement and the severity of crashes, and subsequent injuries.

• Driving over the speed limit:• increases your chances of being involved in a crash• means you have less time to react to avoid a crash• takes longer to stop the vehicle to avoid a crash• increases the severity of injury in a crash.

SometimesBecause of the Law of Conservation of Momentum, energy is neither created nor destroyed. Thus, if specialists know the angles that the incoming vehicles collided at and the weight of the vehicles, they can determine the speeds of the vehicles.

Do the Work

Measuring skid marks:The skid speed is the speed of the vehicle at the beginning of the visible skid mark. This will bea conservative value as the wheels do not lock-up instantly. There is some "shadow skid," alight mark produced as the wheels begin to slow and just before they achieve full lock. Shadowskid and clearly visible skid should be considered as one continuous mark for any given tire.Cars have four tires, two in the front followed directly by the two in the rear. The wheels on mostcars, assuming the brake system is functioning correctly, will tend to lock at nearly the sametime. Current brake design includes pressure limiters that prevent the rear wheels from lockingbefore the front wheels lock.If all four wheels lock at the same time, and the vehicle is skidding in a straight line, the marksfrom the rear wheels will overlap the marks from the front wheels. Rear wheel skid marks canbe identified by the dark center while skid marks from the front wheels can be identified by twodistinct thin lines on the outer edges.If four distinct skid marks can be found, they should be measured individually. To get theaverage skid distance for the vehicle, add the four measurements together and divide by four.This is the "average skid distance." If three skid marks are found, add the three together anddivide by three to get the average skid distance. The same applies to two marks. If only onemark is found, measure the entire length and use this as the skid distance.

Varied Surface Varied Drag

Join me in Speed

Various road surfaces from different initial speeds

I hit a stationary object

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