Stopping Distances Formula Easy to remember.
20 Mph Multiply speed by 2 so 20mph x 2 = 40 feet
30 Mph Multiply speed by 2.5 so 30mph x 2.5 = 75 feet
40 Mph Multiply speed by 3 so 20mph x 3 = 120 feet
50 Mph Multiply speed by 3.5 so 50mph x 3.5 = 175feet
60 Mph Multiply speed by 4 so 60mph x 4 = 240 feet
70 Mph Multiply speed by 4.5 so 70mph x 4.5 = 315 feet
Remembering stopping distances is easy.
Easy Stopping distance formula.
As you can see if you start from 20 mph and multiply by 2 then you get the stopping distances for 20 Mph, then for 30 mph multiply by 2.5 and so on, just start at 20 x 2 and go up by half for each additional 10 mph. so 20mph x2, 30mph x 2.5, 40mph x 3 and so on.
Double these distances for a wet road surface. And for Ice or snow multiply by 10 so in the snow at 30 mph it will take you about 60 car lengths to stop!
Be careful to read the questions correctly on your theory test as you may be asked the thinking distance, the braking distance or the overall stopping distance. Again you can use the same formula, just remember that the thinking distance is always the same as the speed, I.E
20mph thinking distance = 20 ft.
30mph thinking distance = 30 ft.
40mph thinking distance = 40 ft.
And so on.
So at 40 mph the overall stopping distance will be (40 x 3 ) 120 feet so if we deduct the thinking distance which would be 40 feet, this leaves us with the braking distance of 80 feet.
Obviously this wont help much in an emergency, but it is a great way to memorise the figures for your theory test.
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Why is it important to know car stopping distances
Probably more important than learning the quoted stopping distances is knowing just how much further it takes to stop at higher speeds, I.E at 30 mph the quoted stopping distance is 75 feet but add an extra 10 mph and at 40 mph the quoted stopping distance increases by 45 feet, that is more than a 50% increase. At 70 mph it will take you at least the length of a football pitch to stop, and that is if you have quick reactions, good brakes and a dry road surface, in the wet this distance would probably more than double. your thinking distance is the same amount in feet as your speed so even at 40 mph in an emergency your car will travel 40 feet before you even get your foot on the brake!
So now you can see why in dry conditions you need a 2 second space between you and the car in front, and you need to double it to 4 seconds in the wet.
Only a fool breaks the 2 second rule
One of the most important rules to follow while driving is the 2 second rule, in dry conditions you should always have a 2 second time gap between yourself and the vehicle in front. This distance should be doubled to 4 seconds in wet conditions. An easy way to achieve this is to watch when the vehicle ahead of you goes past a fixed object such as a lamp post or traffic sign and say to yourself slowly “only a fool breaks the 2 second rule”. You should be able to finish saying that phrase before you pass the same fixed object, if you pass the object before you finish the phrase then you are too close. In wet conditions it is the same but you need to say the phrase twice to make it 4 seconds.
In icy weather then you need to leave a gap of 20 seconds, or prefefably leave your car at home as it can be very difficult to stop on ice.
For more information on all aspects of road safety you could have a look at the website of the Royal Society of the Prevention of Accidents at Rospa.com. You will find loads of tips and advice on how to stay safe on the roads.
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