Stopping Distance Calculator
Table of contents
Stopping and braking distanceHow do I calculate the stopping distance?What is the driver's perceptionreaction time?Calculating the stopping distance: an exampleFAQsEven if you're not a driver, you'll surely find the stopping distance calculator interesting. From the moment you spot a potentially dangerous situation to the moment when the car comes to a complete stop, it travels a certain distance. You can use this stopping distance calculator to find out how far your car travels in that time, depending on your speed, the slope of the road, and weather conditions.
In this text, we will clarify the difference between the stopping distance and the braking distance. We will also explain how to calculate the stopping distance according to
(the American Association of State Highway and Transportation Officials).Stopping and braking distance
Imagine that you are driving your car on a regular street. Suddenly, you notice a child dart out across the street ahead of you. What happens during the next few stressful seconds?
First of all, some time will pass between the event happening and you perceiving it. This period is called the perception time. During this time, the car continues to move with the same speed as before, approaching the child on the road.
You might think that, as soon as you perceive the event, you hit the brake immediately, but there is always a small delay between the moment you notice the danger ahead and the instant in which you actually start to decelerate. This delay is called the reaction time. The car is still moving with the same speed.
After you start braking, the car will move slower and slower towards the child until it comes to a stop. The distance traveled from the moment you first hit the brake until you come to a complete stop is called the braking distance. The stopping distance, on the other hand, is the total distance traveled since the event began  the sum of distance travelled during perception, reaction, and braking time.
How do I calculate the stopping distance?
The AASHTO stopping distance formula is as follows:
s = (0.278 × t × v) + v² / (254 × (f + G))
where:
s
– Stopping distance in meters;t
– Perceptionreaction time in seconds;v
– Speed of the car in km/h;G
– Grade (slope) of the road, expressed as a decimal. Positive for an uphill grade and negative for a downhill road; andf
– Coefficient of friction between the tires and the road. It is assumed to be 0.7 on a dry road and between 0.3 and 0.4 on a wet road.
This formula is taken from the book "A Policy on Geometric Design of Highways and Streets". It is commonly used in road design for establishing the minimum stopping sight distance required on a given road. With correct parameters, it's a perfect equation for the accurate calculation of the stopping distance of your car. Clearly, it's different than the typical formula used in the speed calculator.
Most of the parameters in the formula above are easy to determine. You can have a big problem, though, when you try to estimate the perceptionreaction time. We'll discuss it now.
What is the driver's perceptionreaction time?
AASHTO recommends the value of 2.5 seconds to ensure that virtually every driver will manage to react within that time. In reality, many drivers are able to hit the brake much faster. You can use the following values as a rule of thumb:
 1 second – A keen and alert driver;
 1.5 seconds – An average driver;
 2 seconds – A tired driver or an older person; and
 2.5 seconds – The worstcase scenario. It is highly probable that even elderly or intoxicated drivers will manage to react within 2.5 seconds.
Calculating the stopping distance: an example
To determine the stopping distance of your car, follow the steps below.

Determine your speed. Let's assume that you're driving on a highway at a speed of 120 km/h.

Decide on your perceptionreaction time. Let's say that you had a good night's sleep (with the help of the sleep calculator) before hitting the road but have been driving for some time now and are not as alert as you could be. You can set your perceptionreaction time to 1.5 seconds.

Input the slope of the road. If it is flat, you can just enter 0%.

Is the road wet or dry? Let's assume it just rained. With a speed of 120 km/h, our braking distance calculator gives us a friction coefficient of 0.27.

Input all parameters into the AASHTO equation:
s = (0.278 × t × v) + v² / (254 × (f + G))
s = (0.278 × 1.5 × 120) + 120² / (254 × (0.27 + 0))
s = 50 + 14400 / 68.6
s = 50 + 210
s = 260
Your car will travel 260 meters before it comes to a stop.
💡 Being able to stop in time is crucial to road safety. If you visit the car crash calculator, you can see the potential impact of a collision.
What is the stopping distance on a dry road?
On a dry road the stopping distances are the following:
Speed  Stopping distance 

10 mph  41 ft (13 m) 
20 mph  93 ft (28 m) 
30 mph  153 ft (47 m) 
40 mph  223 ft (68 m) 
50 mph  302 ft (92 m) 
60 mph  392 ft (120 m) 
70 mph  491 ft (150 m) 
What is the stopping distance on a wet road?
On a wet road the stopping distances are the following:
Speed  Stopping distance 

10 mph  43 ft (13 m) 
20 mph  107 ft (32 m) 
30 mph  195 ft (59 m) 
40 mph  311 ft (95 m) 
50 mph  457 ft (139 m) 
60 mph  636 ft (194 m) 
70 mph  849 ft (259 m) 
What is the stopping distance for a car traveling at 50 kph?
The answer is a bit less than 50 m. To get this result:

We assume the road is flat and dry.

Moreover, we assume an average perceptionreaction time of 2.5 seconds.

We apply the stopping distance formula, which (under our assumptions) reads:
s = (0.695 × v) + (v² / 177.8)
.Here,
v
denotes the car's speed in km/h. 
Plugging in
v = 50
, we get48.81 m
.