Even 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 AASHTO (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 – Perception-reaction 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; and
  • f – 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.

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 perception-reaction time. We'll discuss it now.

What is the driver's perception-reaction 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 worst-case 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.

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

  2. Decide on your perception-reaction time. Let's say that you had a good night's sleep 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 perception-reaction time to 1.5 seconds.

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

  4. 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.

  5. 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.

FAQ

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:

  1. We assume the road is flat and dry.

  2. Moreover, we assume an average perception-reaction time of 2.5 seconds.

  3. 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.

  4. Plugging in v = 50, we get 48.81 m.

Bogna Szyk
Speed
mph
Perception - reaction time
sec
Grade
%
The road is...
dry
Stopping distance
ft
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