About how long does it take for a driver to react to a hazard and put his/her foot on the brake?

The simple truth about speeding is: the faster you go, the longer it takes to stop and, if you crash, the harder the impact. Even small increases in speed could have severe consequences. If a pedestrian steps out into the path of an oncoming vehicle which is speeding the difference could be a matter of life or death.

Table of Contents Show

The reaction distance is affected by
The reaction distance can be decreased by
The reaction distance can be increased by
The braking distance is affected by
Calculate the braking distance
Stopping distance

[Speaking - Professor Barry Watson Direct CARRS-Q, QUT]

The simple truth about speeding is that the faster you go the longer it takes to stop.

Here’s a comparison.

This is a car travelling at 60km/h and braking suddenly.

Here’s the same car, travelling at 67km/h and braking at the exact same point.

This time the car hits, still travelling at 30km/hr. The difference could be a matter of life or death.

In an emergency, the average driver takes about 1.5 seconds to react. Stopping distances increase exponentially the faster you go.

You can also access this infographic information in text form.

The stopping distances on the infograph are calculated based on the following assumptions:

In an emergency the average driver takes approximately 1.5 seconds to react
A modern vehicle with good brakes and tyres, after braking, is capable of stopping at approximately 7 m/s2.
A dry road that is sealed and level enables good friction between the tyres and the road to help stop the vehicle sooner. Scientifically, it has a coefficient of friction of approximately 1.
A wet road that is sealed and level has less friction between the tyres and the road which increases the stopping distance of a vehicle. Scientifically, the coefficient of friction of approximately 0.7.

The stopping distances in the graph are generic and may be influenced by a number of driver, vehicle and environmental factors:

Driver factors	Vehicle factors	Environmental factors

	Type and condition of brakes
Impairment due to alcohol and drugs	Type and condition of tyres, including tyre pressure
	Safety features fitted to the vehicle for example ABS, ESC, EBA, etc.
Driver age and experience
Hazard perception ability	Towing a trailer or carrying a heavy load

Last updated: 14 November 2016

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The reaction distance is the distance you travel from the point of detecting a hazard until you begin braking or swerving.

The reaction distance is affected by

The car’s speed (proportional increase):
- 2 x higher speed = 2 x longer reaction distance.
- 5 x higher speed = 5 x longer reaction distance.
Your reaction time.
- Normally 0.5–2 seconds.
- 45–54 year-olds have the best reaction time in traffic.
- 18–24 year-olds and those over 60 have the same reaction time in traffic. Young people have sharper senses but older people have more experience.

The reaction distance can be decreased by

Anticipation of hazards.
Preparedness.

The reaction distance can be increased by

Formula: Remove the last digit in the speed, multiply by the reaction time and then by 3.

Example of calculation with a speed of 50 km/h and a reaction time of 1 second:

50 km/h ⇒ 5
5 * 1 * 3 = 15 metres reaction distance

Formula: d = (s * r) / 3.6

d = reaction distance in metres (to be calculated).
s = speed in km/h.
r = reaction time in seconds.
3.6 = fixed figure for converting km/h to m/s.

Example of calculation with a speed of 50 km/h and a reaction time of 1 second:

(50 * 1) / 3.6 = 13.9 metres reaction distance

Drive slowly or you might not even have time to react to oncoming traffic.

The braking distance is the distance the car travels from the point when you start braking until the car stands still.

The braking distance is affected by

The vehicle’s speed (quadratic increase; “raised to the power of 2”):
- 2 x higher speed = 4 x longer braking distance.
- 3 x higher speed = 9 x longer braking distance.
The road (gradient and conditions).
The load.
The brakes (condition, braking technology and how many wheels are braking).

Calculate the braking distance

It is very difficult to achieve reliable calculations of the braking distance as road conditions and the tyres’ grip can vary greatly. The braking distance may for example be 10 times longer when there is ice on the road.

Conditions: Good and dry road conditions, good tyres and good brakes.

Formula: Remove the zero from the speed, multiply the figure by itself and then multiply by 0.4.

The figure 0.4 is taken from the fact that the braking distance from 10 km/h in dry road conditions is approximately 0.4 metres. This has been calculated by means of researchers measuring the braking distance. Thus, in the simplified formula, we base our calculations on the braking distance at 10 km/h and increase it quadratically with the increase in speed.

Example of calculation with a speed of 10 km/h:

10 km/h ⇒ 11 * 1 = 1

1 * 0.4 = 0.4 metres braking distance

Example of calculation with a speed of 50 km/h:

50 km/h ⇒ 55 * 5 = 25

25 * 0.4 = 10 metres braking distance

Conditions: Good tyres and good brakes.

Formula: d = s2 / (250 * f)

d = braking distance in metres (to be calculated).
s = speed in km/h.
250 = fixed figure which is always used.
f = coefficient of friction, approx. 0.8 on dry asphalt and 0.1 on ice.

Example of calculation with a speed of 50 km/h on dry asphalt:

502 / (250 * 0.8) = 12.5 metres braking distance

Stopping distance

Stopping distance = reaction distance + braking distance

It is summer and the road is dry. You are driving at 90 km/h with a car with good tyres and brakes. You suddenly notice a hazard on the road and brake forcefully. How long is the stopping distance if your reaction time is 1 second?

The stopping distance is the reaction distance + braking distance. First we calculate the reaction distance:

90 km/h ⇒ 9
9 * 1 * 3 = 27 metres reaction distance

Then we calculate the braking distance:

90 km/h ⇒ 9
9 * 9 = 81
81 * 0.4 = 32 metres braking distance

Now both distances are combined:

27 + 32 = metres stopping distance

The different methods provide different answers. Which should I use?
– Use whichever you wish. The differences are so small that they will not affect your theory test, as the margins between the alternatives are quite large.

So if the alternatives are 10, 20, 40, 60, it does not matter if you get 10 metres with one method and 12.5 metres with another – both are obviously closest to 10, which is thus the right answer.

Free demo – try the questions

When admiring a new car, no one ever asks the owner how well its stops....just how well does it goes

Many drivers, drive in a false belief that if the car in front suddenly started braking, they would react and brake and end up stopped the same distance apart.

The total stopping distance of a vehicle is made up of 4 components.

Human Perception Time
Human Reaction Time
Vehicle Reaction Time
Vehicle Braking Capability

The human perception time; is how long the driver takes to see the hazard, and the brain realize it is a hazard requiring an immediate reaction. This perception time can be as long as ¼ to ½ a second.

Once the brain realizes danger, the human reaction time is how long the body takes to move the foot from accelerator to brake pedal. Again this reaction time can vary from ¼ - ¾ of a second.

These first 2 components of stopping distance are human factors and as such can be effected by tiredness, alcohol, fatigue and concentration levels. A perception and reaction time of 3 or 4 seconds is possible. 4 seconds at 100 km/hr means the car travels 110 metres before the brakes are applied.

Once the brake pedal is applied there is the vehicles reaction time which depends on the brake pedal free-play, hydraulic properties of the brake fluid and working order of the braking system.

This is why the tailgating car usually cannot stop, when the brake light came on in the car in front, this driver had already completed the perception, human and vehicle reaction periods. The following driver was perhaps 1 second to late in applying the brakes. At 100km/hr the car required 28 metres further to stop.

The last factor than determines the total stopping distance is the cars braking capability which depends on factors such as;

the type of braking system,
brake pad material,
brake alignment,
tyre pressures,
tyre tread and grip,
vehicle weight,
suspension system,
the co-efficient of friction of the road surface,
wind speed,
slope of road,
surface smoothness
the braking technique applied by the driver.

Worth noting is that from 50 to 100 kph the braking distance of a car will increase from 10 metres to 40 metres. When you double the speed of a car braking distance quadruples.

This is based on the laws of physics. When a car is moving it has kinetic energy, ½mv2. When the velocity doubles the kinetic energy quadruples. The braking capability does not increase when driving faster, there are no reserves of friction. As such in any vehicle when your speed doubles braking distance is four times larger.

BRAKING DISTANCE FROM 100 km/hr (real world testing)

The table below lists the BRAKING DISTANCE of various cars from 100 km/h. These cars were tested at different locations on different days.

Be careful comparing results as test results can vary depending on many factors including the road surface, how the speed was measured (as various cars have differing speedometer accuracies), the tyre pressures, fuel load and whether the car had only the driver or had additional passengers.

MAKE/MODEL	DISTANCE (m)	SOURCE
Alfa MITO	37.61	Motor Magazine (Aust)
Alfa Giulietta QV	37.80	Motor Magazine (Aust)
Audi A5 Sportsback	37.62	Motor Magazine (Aust)
BMW 123D Hatch	37.95	Motor Magazine (Aust)
BMW 330D Coupe	36.63	Motor Magazine (Aust)
Chrysler 300C	38.72	Motor Magazine (Aust)
Holden VE Commodore SV6	39.86	Motor Magazine (Aust)
HSV GXP	37.76	Motor Magazine (Aust)
HSV GTS (WP tuned - 2011)	38.31	Motor Magazine (Aust)
Nissan GTR (R35 - 2011)	32.75	Motor Magazine (Aust)
Porsche 911 Turbo S (2011)	39.62	Motor Magazine (Aust)
Renault Megane RS250	36.34	Motor Magazine (Aust)
Renault RS Clio 200	36.43	Motor Magazine (Aust)
Subaru Impreza WRX	37.38	Motor Magazine (Aust)
Suzuki Alto	43.56	Motor Magazine (Aust)
W Golf GTD	37.58	Motor Magazine (Aust)
VW Golf R	39.57	Motor Magazine (Aust)
VW Golf GTI	39.36	Motor Magazine (Aust)
Volvo C30 TS	39.05	Motor Magazine (Aust)

BRAKING & STOPPING DISTANCE CHART (government data)

This is a chart issued by a transport department. It shows reaction distance, braking distance and total stopping distance in a convenient diagram. However the devil is most definetly in detail.

This diagram suggests 60 metres of braking distance will be required by a car at 100 km/h. Resulting in a total stopping distance of 88 metres. There are many of these diagrams issued around the world and most tend to have a total stopping distance at 100 km/h within the range of 80 metres to 94 metres.

Real world testing data (see above table) suggests a modern car only requires a braking distance of less than 40 metres.

Now it is conceded that "worst-case scenarios" need to be considered as we all don't drive sportscars. However I suggest many government diagrams are at best "out-of-date" and at worst "exaggerated". Keep in mind any exaggeration is magnified in the government charts as the speeds increase.