“One, two, three” - it is just one second and three of them will save your life

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Failing to keep a safe distance between vehicles is one of the most frequent causes of road accidents. In some countries traffic regulations define the distance between vehicles in meters. For instance in Germany this distance shouldn’t be shorter that half of the vehicle speed (at 100 km/h - 50 m), in Czechia, for vehicles heavier than 3.5 tons it is the vehicle length + 10 m.

Let's take a look at behaviours of drivers on the road: a line of cars on the motorway often means that the distance between vehicles is short, often not more than a few meters. Will 10 meters be enough to brake between on obstacle at the speed of 70 km/h? Definitely not. So why such a practice is not used? The most frequent answer is no awareness of the laws of physics and poor imagination of drivers.

 

Why do short distances between vehicles cause accidents?

The distance should be sufficient to stop the vehicle in an emergency situation and such situations include a traffic jam, an accident, a vehicle suddenly stopping in front of us. At the speed of 75 km/h the driver often spots an obstacle on the road approximately 36 meters before the vehicles hence they will crash into it at the speed of 53 km/h - see the drawings below.

Stopping distance

Braking distance has many elements - spotting the obstacle, taking a decision to brake (reaction time), time when the braking system starts working and the correct braking distance.

Reaction time depends on several factors: the speed but also the physical and mental condition of the driver - their distraction, fatigue, alcohol and medications, Reaction time is usually between 0.5 to 2 seconds and the average value is 1 second. The proper braking distance depends on the driver's skills, conditions on the road (rain, snow) as well as the mass of the vehicle which depends on the number of passengers or the cargo weight. Also an important role is played by the tyre pressure and technical condition of tyres (i.e. thread quality) and the brakes. 

In the rain braking distance is on average twice longer and ice may lengthen it even 10-fold.  Braking distance of a commercial goods vehicle (weighing approximately 40 tons) is three time longer than that of a car.

Average braking distance

At the speed of 50 km/h, within 1 second the vehicle will cover 13.89 m while at a speed of 90 km/h it will be 25 m. The diagram below shows the braking distances for a car driven by a focused driver in good road conditions (dry road surface).

Average braking distance

At the speed of 50 km/h, within 1 second the vehicle will cover 13.89 m while at a speed of 90 km/h it will be 25 m. The diagram below shows the braking distances for a car driven by a focused driver in good road conditions (dry road surface). 

 “I don't want to have an accident” is the motto of defensive driving which allows for predicting incidents on the road. While driving we should assume the possibility of emergency braking hence we should take into account the distance we need to stop the car in specific road conditions. In line with proactive driving rules, one should keep the safe distance of at least 3 seconds for cars and 6 seconds for trucks. If road conditions are unfavourable (rain, snow, ice), safe distance should be longer.

Calculating the 3-second distance

Saying the words “one, two, three” takes one second. So three seconds means saying this sequence three times. In order to measure the distance of 3 seconds, start counting “one, two, three, one, two, three, one, two, three” when the car in front of you passes a specific landmark on the road (a tree, a road sign, a mile marker, etc.). If your car passes that landmark before you finish counting 3x one, two, three, you should slow down and increase your distance.

To sum up, you have a choice between maintaining the distance of 3 seconds, anticipating problems on the road, i.e. avoiding the risk of accident or driving too close with a high probability of an accident.

 

Check out the app for measuring braking distance:

www.brake.org.uk/facts-resources/21-resources/1339-stopping-distances-calculator

20.02.2018
Grażyna Łukasik

Grażyna Łukasik

SHE Manager

Runs SHE projects (Safety, Health and Environment)in Raben Group. Specializes in occupational health and safety as well as in ergonomics of workstations. A DGSA Advisor since 2003.

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