Stopping Distances for Cars and Lorries.

Speed	Distance to Stop	Feet per Second	Car lengths
20 mph	12m 40ft	29.4 ft/sec	4
30 mph	23m 75ft	44.1 ft/sec	6
40 mph	36m 120ft	58.8 ft/sec	10
50 mph	53m 175ft	73.5 ft/sec	14
60 mph	73m 240ft	88.2 ft/sec	20
70 mph	96m 315ft	102.9 ft/sec	26

In traffic, you should always choose the lane that allows the maximum possible stopping distance.
It is possible to calculate stopping distance mathematically using a graph if you know approximately how much time it takes to stop at a certain speed. Lets say you're going 60 mph.

How long does it take for your car or truck to stop at 60 mph? The obvious way to test it is to look at your watch and slam your brakes on, then count how many seconds it takes to stop.

Lets pretend you are driving an eighteen-wheeler and it takes six seconds to stop at 60 mph. That works out to 10 mph of slowing down for each second you apply the brakes. At 10 mph. per second, every second you put the truck's brakes on will slow you down by 10 mph. By knowing how much time it takes to stop at a certain speed, you can calculate very precisely what the stopping distance is going to be at any speed.

First let us calculate what the stopping distance is at 10 mph. 10 mph is equal to about 14 feet per second. There are 5,280 feet in a mile and 3,600 seconds in an hour, so to convert miles per hour to feet per second you just multiply your mph by 5,280 and divide by 3,600. You will go fourteen feet in one second at ten miles per hour. Since you're slowing down from ten mph and ending up at zero, you will only average 5 mph or 7 ft. per second while you have your brakes on. How many feet do you travel in one second at 7 feet per second? Obviously, you travel seven feet, so your stopping distance is seven feet.

To calculate what your stopping distance is going to be from 20 mph you first have to calculate the distance you are going to travel as you slow from 20 down to ten. Your average speed is going to be 15 or 22 feet per second so the total stopping distance is 22 feet plus the seven feet when you slow from 10 down to zero or 29 feet.

With air brakes, that's not all there is to it.

You also have to take into account the air brake lag. It takes about a half-second for air brakes to activate because pressure must build in the lines before the actuators will start to move.

At 10 mph or 14 feet per second, you will travel seven feet in a half second so the stopping distance from the time you step on the brake pedal will be 7+7 or 14 ft. That is seven feet of air brake lag and seven feet of stopping distance added together. At 20 mph the total distance will be 22+7+15 or 44 feet including 15 ft. of air brake lag. What you can immediately see from this is that if you double your speed, you increase your stopping distance considerably. 14 feet to 44 feet is more than three times.

So if you are going 10 mph and you increase your speed to 20, your stopping distance is going to be three times what it was a 10 mph. You can continue calculating and find out what your stopping distance will be at 30 mph. At 30 mph you will add 38 feet, so your stopping distance will be 7+22+38 which works out to 88 feet when you include 22 feet of air brake lag. If you increase your speed from 20 to 30, you double your stopping distance.

Looking at these calculations its easy to see why we have speed limits. If you increase your speed from 10 mph to 20 mph, you triple your stopping distance. If you increase your speed from 20 mph to 30 mph, your stopping distance is six times greater.

SPEED	STOPPING DIST	SAFE FOLLOWING DIST	SECONDS
10 mph	14 ft	44 ft	3.0 sec
20 mph	44 ft	100 ft	3.5 sec
30 mph	88 ft	175 ft	4.0 sec
40 mph	146 ft	263 ft	4.5 sec
50 mph	220 ft	365 ft	5.0 sec
60 mph	306 ft	482 ft	5.5 sec
70 mph	409 ft	614 ft	6.0 sec

 

SPEED	STOPPING DIST	SAFE FOLLOWING DIST	SECONDS
20 mph	14 ft	72 ft	2.5 sec
40 mph	58 ft	174 ft	3.0 sec
60 mph	130 ft	305 ft	3.5 sec
80 mph	230 ft	464 ft	4.0 sec

This table shows stopping distances. (calculations are different from the top table, fig. 1 and Fig. 2-3, you will notice the distance to stop is shorter on fig. 2-3, Average speed reduction time is different in the two calculations). It also shows the minimum safe following distance.

A vehicle following another needs more room to get stopped because it takes about two seconds longer for the person following to get his foot off of the accelerator and onto the brake, even if the two vehicles have identical stopping distances.

The safe following distance is equal to the regular stopping distance plus the distance the vehicle travels in the two seconds it takes for the driver to react and move his foot at each given speed.

Following another vehicle at a distance less than this would not be safe because if an object were to fall off the vehicle in front, you would not be able to stop in time to avoid colliding with it. The "seconds" column is the number of seconds it takes to cover the distance in the safe following distance column at each given speed. This makes it easy to measure following distance while you are driving.

For instance, it takes three seconds to go 44 feet at 10 mph and it takes 5.5 seconds to go 482 feet at 60 mph. Measuring safe following distance in seconds is convenient because you can easily count how much time elapses between the time the vehicle in front passes a landmark such as a road sign until you pass the same landmark yourself. Notice that the number of seconds needed to be safe increases with speed from three seconds for cars at 40 mph to four seconds at 80 mph.

Note that the table does not make a distinction between single and multi-lane highways. The safe following distance is 305 feet at sixty miles an hour regardless how many lanes a highway may have or what lane the other traffic may be using. This is because if an object falls off a long flatbed eighteen-wheeler, it is more likely to fall to the side than to the rear.

The following table shows typical stopping distances for Motorbikes