Calculating the braking distance – what affects the braking distance?

The braking distance is an important value in road traffic and is defined as the necessary distance for a car to reduce its actual speed to the desired speed. The braking distance is often underestimated, causing drivers to ignore the safe distance at high speed. Knowing how to calculate the braking distance and which factors have an impact on it, can save lives. Therefore we would now like to shed a light on its main aspects.

Braking distance, stopping distance, safe distance

If you recently visited driving school you will certainly remember these concepts, as they are an elementary part of the driving test.
Understanding their meaning and knowing how to correctly apply the correct formulas is recommended, considering that these are so-called thumb formulas, i.e. simplified approaches.

The braking distance is the so-called key issue, taking into account two versions, namely the normal braking manoeuvre and the emergency stop. In a dangerous situation, the brake pedal is fully applied, increasing the braking power and shortening the braking distance. Despite the danger, many drivers do not trust themselves to fully apply the brake pedal. This must be practised in regular intervals. Depending on the situation, it being a normal braking manoeuvre or an emergency stop, the applicable formula changes. To normal braking manoeuvres applies the following:

Please note: these formulas only apply on condition of a dry road and proper tyres with sufficient profile. The stopping distance includes the braking distance, but considers the total braking action taking into account the reaction time, response time and device response time. Basically, the stopping distance indicates the amount of road necessary to come to a complete stop in an emergency situation. The reason why you cannot simply use the calculated braking distance is evident: between the time necessary to acknowledge the danger and to react, the car drives on at constant speed, even if only for tenths of seconds. The difference becomes clear when the values are compared:

At 70 km/h the result from our previous example is a braking distance of 24.5 m, but a total stopping distance of 45.5 m, almost twice as long. This is one of the reasons why the stopping distance is often underestimated.

The safe distance is the last value in this context. It should be chosen in such a way, that it is at least as long as the reaction distance. The thumb rule used to be “half the speedometer”. This rule is still true, but only if the road is dry and there is a clear view of the car in front. A new formula includes these factors and is as follows:

For factors location and weather condition different values apply. The factor (location) in a built-up area is 1 and in the open country or on the motorway 2. For the factor weather condition a dry road has the value 1, rain 1.5 and snow 4. The result at 70 km/h outside the built-up area on a dry road is a required safety distance of 35 m, in this case indeed representing half of the speedometer and even more than the calculated reaction distance.

What should be taken into account for safe braking distance?

Several factors can lengthen or shorten the braking distance, such as the technical properties of the car as well as the local circumstances. The former can actively be influenced, the latter can’t. A driver should observe the following:

1. The braking system – The braking system has a decided influence on the braking function and therefore on the braking distance as well. It must be in impeccable condition at all times and maintained regularly. Excessively worn braking discs and brake pads reduce the friction coefficient and can in extreme cases double the braking distance. This doesn’t happen overnight, but gradually, and could mean the difference between an accident and a near-accident. The MOT inspection requires a check of the brakes every two years. Drivers spending a lot of time on the road should have their braking systems checked – and if necessary replaced – every 30.000 kilometres. The difference between drum brakes and disc brakes can be ignored with respect to braking force.

2. The tyres – The tyres have a considerable influence on the braking distance, and here the driver can do his part as well. The tyre profile is decisive. Theoretically a tyre without profile would have the better braking performance. For this reason, Formula 1 cars are equipped with so-called “slicks”. However, as soon as the road is no longer optimal, i.e. muddy or wet, this changes everything. A 1.6 mm profile depths is mandatory, although experts recommend 3 mm. Only then a consistent braking distance is guaranteed. The use of wrong tyres constitutes the highest risk: driving on snow with summer tyres or during summer with winter tyres. In the first example braking is often no longer possible. The second example can cause a flat tyre and consequently loss of control over the car.

3. Weather and on-road conditions – No matter how much you wish for a dry and well-paved road, these conditions cannot be influenced. However, you can adapt your driving style to weather and other conditions by increasing the safe distance and reducing speed. Many drivers do not realise how the braking distance changes under different weather conditions. Here is an example:

The differences are unbelievable. Particularly in winter it is very important to drive in a careful and anticipating manner as an emergency stop is almost impossible.

4. The slope – Unless you live in San Francisco this condition is quite unusual in urban traffic, nevertheless it may be a factor on holidays or during sightseeing tours. Driving up a slope shortens the braking distance, whereas coming down the hill would cause the braking distance to be longer. Another formula applies, but it is considerably more complicated than the usual thumb rules. It is however important to remember that coming down a hill you need to increase your safety distance.

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Progressive and regressive braking techniques

Two words often mentioned in connection to the braking distance and most certainly in the driving school refer to the braking method, more specifically progressive and regressive braking. Both have advantages as well as disadvantages, and driving instructors do not have a clear preference for either one of them.

– Progressive braking is done by initially braking gently and then applying the pedal stronger and stronger in a smooth transition until the car comes to a halt. A typical feature of this braking method is the typical “jolt” of the car shortly before it stops, because the driver often has to step on the pedal harder to prevent driving against the bumper of the car in the front. This method is the more popular one, although it is actually less preferable as it requires plenty of attention and leads to inconvenient jolts.

– Regressive braking means initially stepping strongly on the pedal for an immediate speed reduction, and then smoothly and safely rolling forward, ultimately coming to a halt. For beginners this method is recommended as it practices applying the pedal strongly. The speed is less at the initiation of the braking manoeuvre and therefore the risk of dangerous situations is reduced. The typical “jolt” at the end does not occur at all.
Only disadvantage: if you initially step on the brake pedal too hard, you might possibly need to accelerate again to generate enough energy for rolling forward.

Conclusion

If you pay attention to the subject of braking distance and its influencing factors, not only can you learn a lot, but also become a better driver. What is essential is not the excellent knowing of the formulas by heart, but developing an intuition for situations in which the braking distance is worse and when you need to be especially attentive. The technical aspect cannot be underestimated either. Stipulated inspection intervals are no luxury and ignoring them not only causes high fines but puts your life as well as that of others in danger. 

Weblinks: MOT
Foto: standret, mstanley, Iaroslav Neliubov, zlikovec, Tikhomirov Sergey, Earl D. Walker / shutterstock.com