What is the difference between stopping distance and braking distance?
For those who don't have time, let's get to the point, braking distance is a component of stopping distance:
Stopping distance = reaction distance + braking distance
There are several factors that influence the stopping distance, we will detail them in the rest of this article, but the most important is the speed of the vehicle.
It is possible to quickly estimate the stopping distance of a vehicle as a function of its speed by calculating the square of the number of tens of the speed.
At 50 km/h, the stopping distance of a vehicle will be 25m (5x5=25).
You will find a summary table of stopping distances according to the current French speed limits at the end of this article 👇.
Before we get to that, let's get into the details of the components of stopping distance: reaction distance and braking distance.
Reaction distance: the distance covered during the reaction time
Before deciding to apply the brake, the driver must first perceive a danger and, above all, qualify it as such. Once the danger is perceived, the driver of the vehicle will analyze the situation, validate that it is indeed a danger and that it is necessary to brake, decide to brake and, finally, press the brake.
During this time, the distance covered by the vehicle is called the reaction distance and is a component of the stopping distance.
The average reaction time of a healthy, physically fit person is considered to be about one second.
How to calculate the reaction distance
The distance traveled during the reaction time (= 1 second) can be calculated as follows:
Reaction distance (in m) = (speed x 1000) / 3600
For example, at 50 km/h, the distance traveled during the reaction time is 14 m (50000/3600 ≈ 14).
A faster calculation is allowed by multiplying the tens of the speeds by 3: at 50 km/h = 5x3 = 15 m.
Factors influencing the reaction distance
The driver of the vehicle is the only variable capable of influencing the reaction distance, so the factors are mainly related to his physical condition such as:
- His health status;
- His state of fatigue;
- If driving under the influence of alcohol or drugs ;
- His level of experience.
Speed does not affect the reaction time, but it does increase the distance covered during this period. For example, at a speed of 100km/h, keeping a reaction time of one second, the car will travel 30m before the driver hits the brake.
It is known that poor weather conditions can significantly reduce visibility and therefore delay the perception of a hazard. The weather factor influences the speed at which the driver of the vehicle will perceive the danger and not the reaction time.
Braking distance: the distance between the start of braking and the complete stop of the vehicle
Once the action of applying the brake has been carried out, the human factor is no longer involved in bringing the vehicle to a stop. The distance that the vehicle will travel between the start of braking and its complete stop, called the braking distance, depends only on factors specific to the vehicle and its environment.
Speed, the primary factor affecting braking distance
If you are comfortable with physics, the braking distance is calculated using the following formula:
Braking distance (Df) = v²/(2gf)
- v = speed (converted to meters per second)
- g = acceleration constant (9.81)
- f = road adhesion coefficient (average value = 0.7)
For 50km/h (i.e. 8.4 m/s), the braking distance will then be about 14.27 m (Calculation: Df = 8.4² / 2x9.81x0.7).
To better understand the influence of the speed on the braking distance, let's do the same calculation for a vehicle running at 100km/h (28 m/s): Df = 28² / 2x9,81x0,7 = 57 m
➡️ So doubling his speed is like multiplying his braking distance by 4 (57/14.27=4)!
If you prefer a simplified version, simply subtract the reaction distance from the stopping distance to get the braking distance.
Braking distance = Stopping distance - Reaction distance
For 50 km/h : Braking distance = 25 - 14 = 11m
Note: it is possible to calculate a quick approximation of the braking distance (reduced vs. reality) by dividing the square of the tens of the speed by 2 (for 50 km/h: 5x5/2 = 12,5m)
Other factors that can affect braking distance
In addition to speed, there are other factors that affect the car, such as its weight. The heavier the vehicle, the longer it will take to stop.
The quality and proper maintenance of equipment also plays a major role in reducing braking distance. This is the case of tires or brakes which, if they are in bad condition, will considerably increase the braking distance. Shock absorbers can also play an important role depending on the terrain.
Finally, bad weather conditions will increase the braking distance:
- On wet ground, the braking distance is multiplied by 1.5;
- Low temperatures harden conventional tires, which lowers their performance;
- Ice or snow can increase braking distances by up to 10 times.
Note: The above calculation is valid only for a vehicle in perfect working order.
Some examples according to the current speed limits
It is important to remember that the "countdown" of the stopping distance only starts when the driver has perceived the danger. It is therefore essential to correctly signal a danger or to maximize your chances of being perceived by other road users when you are walking or cycling, for example.
Here is a summary of the stopping distances according to the main speed limits in force in France if the driver is in good physical condition and if the weather conditions are good.
Do you want to know how to maximize your visibility and therefore your safety when you travel at night? Check out our article on this topic: