Let us examine how the vehicle speed affects the number of accidents and the likelihood of death on the road, and focus on the interaction between vehicles and pedestrians as two “most opposite” traffic participants.
Speed ??and the probability of an accident
Let’s consider the stopping distance. It can be calculated if you know the driver reaction time and the stopping distance of a vehicle after the brakes have been applied.
The average reaction time is 1 second. If the speed increases, the distance travelled per 1 second also increases. The distance traveled from the moment when the brakes were applied to the moment when the car came to a full stop is proportional to the square of the speed of the car. When increasing the speed from 50 km/h to 80 km/h, the braking distance increases by 2 times. So it becomes harder to avoid hitting.
The fact that wet conditions increase the stopping distance by 25% should also be taken into account. That is, the braking distance of a vehicle driving 60 km/h on a wet asphalt road is equal to the one of the vehicle driving 70 km/h on a dry road.
When driving 80 km/h the reaction distance is about 22 meters. And on a dry road the driver will need a minimum of 36 meters to stop the car completely.
If a child runs out into the road at a distance of 36 meters from the car, it will almost certainly die if the initial speed of the vehicle will be 70 km/h, it will be injured at a speed of 60 km/h, and at 50 km/h, the driver will most likely be able to avoid the collision.
But if a child runs out at 15 meters in front of the car, it will be fatally injured in most cases, even if the vehicle speed is 50 km/h.
You can calculate the braking distance and the braking time, as well as the brake delay at a certain distance under different conditions (initial speed, reaction time, type of coating) using the calculator on this German site: http://www.auto-und-verkehr.de/bremswegrechner/
Speed and frequency of road accidents
The design and functional properties of roads affect strongly the relationship between the speed and the frequency of accidents, for example, presence and type of intersections, presence of pedestrians and cyclists.
The risks of accidents and the influence of speed become more crucial in difficult road conditions.
For example, highways are simply organized and carry little risks of accidents. On the contrary, urban roads have a more complex organization increasing the risk of an accident.
The main victims of road accidents in urban areas are pedestrians, cyclists and motorcyclists. The main contributing factors are the differences in their speed and weight.
A research comparing risks from speed limit violations and the ones from the drunk driving was conducted in South Australia. It was agreed that relative risks are equal to 1 (one) at 60 km/h and BAC equal to 0 ppm.
Relative risks begin to rise sharply at 70 km/h. Increasing the basic speed by only 10 km/h the risks grow as if your BAC were 0.8 ppm and your speed – 60 km/h.
Influence of an inhomogeneous traffic flow on the number of accidents
Vehicles driving at different velocities in the traffic flow contribute to the increase of the amount of overtaking and, consequently, to a higher accident risk. Large speed discrepancies between vehicles in a traffic flow certainly contribute to an increase in fatal road accidents, both in the city and out of town.
Generally, the reduction in speed limits results in a decrease in speed discrepancies, and consequently, in the number of fatal cases.
The frequency of accidents grows by 10-15%, when exceeding the speed limit by 1 km/h. When exceeding the average speed of the traffic flow by more than 10 km/h, the number of accidents begins to rise sharply on city roads. For out-of-town roads, the increase in accidents is not so crucial in this case.
The chart also shows that the decrease in individual vehicle speed, relative to the average speed of the traffic, does not lead to an increase in the number of accidents.
Influence of the vehicle speed on the severity of an accident
Even if speeding is not the main cause of an accident, its severity level depends heavily on the speed of the vehicles at the moment of an impact. Approximate dependence of the number of severe and fatal accidents when changing the vehicle speed is shown in the chart.
The increase in speed by 10% leads to an increase in the number of all accidents by 21%, and consequently to the increase in the number of severe and fatal accidents by 33%, and in the number of fatal accidents – by 46%. The decrease of the speed by 10% results in a reduction in the number of these types of accidents by respectively 19%, 27% and 34%.
The situation highly depends on the type of a road and on speed limits set on this road. The chart below shows the increase in road accidents for different vehicle speeds when changing them by 1 km/h.
As you can see from the table, speed variations have the most serious impact on the severity of accidents on roads with low speed limits. These are urban roads, of course.
A group of the road users involved in an accident also affects strongly the severity of the accident. Pedestrians, cyclists and motorcyclists have a high risk of serious injury or death as they are not protected. They do not have a metal frame, seat belts and airbags.
The probability of death of a pedestrian in an accident grows proportional to the increase in a vehicle speed. Investigations have shown that 90% of pedestrians survive in a collision at a speed of 30 km/h, while the collision at 50 km/h leads to the death of 80% of pedestrians.
However, the driver and the passengers do not get injured almost at all.
Influence of the vehicle speed on the visibility
When increasing the speed, the driver visibility decreases significantly. This results from human physiology. Thus, the driver is not able to correctly predict the situation at high speeds in urban areas, as he/she doesn’t have full information about the road conditions.
At 40 km/h, the driver’s visual field is 100 degrees. This allows you to see the obstacles on the road, as well as to control the situation on the right and on the left sides of the road. At a speed of 130 km/h, the visual field is 30 degrees or even less, what reduces greatly the ability of a driver to estimate the potential risk.
High speed causes the third part of all road accidents. Additionally, high speed exacerbates the severity of the accidents that have occurred for other reasons.
The vehicle speed impacts the accidents in the cities most seriously where several groups of road users interact with each other: cars, pedestrians, cyclists.
There is a vehicle speed that gives no chance to a pedestrian to stay alive. Only 50% of pedestrians survive a collision at a speed of 45 km/h.
It’s very important to control the enforcement of reasonable speed limits, as well as to minimize the speed discrepancies in the traffic flow.