Stopping Distances

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Introduction

When driving it is vital that you maintain a decent separation distance from the vehicle in front of you.

This distance will depend on your speed, the road type and of course the prevailing weather conditions. Below are some guidelines.

Separation Distances

Far too many accidents are caused by drivers getting too close to the vehicle in front.

It´s essential that every driver is able to judge a safe separation distance in all road, traffic and weather conditions. The safety of you and your passengers depend on it.

How far should you keep from the vehicle in front? Ideally, you should be no closer than the overall stopping distance that corresponds to your speed.

In heavy, slow-moving urban traffic that might not be practicable, without wasting valuable road space. However, even then, the gap should never be less than your thinking distance and much more if the road is wet or slippery.

Remember that your overall stopping distance is the only really safe gap and anything less is taking a risk!

A reasonable rule to apply in good dry conditions is a gap of one metre for each mph of your speed. For example, at 55mph a gap of 55 metres. in bad conditions leave at least double the distance. A useful technique for judging one metre per mph is to use the "Two-Second Rule".

The Two-Second Rule

In good dry conditions an alert driver, who is driving a vehicle with first class tyres and brakes, needs to be at least two seconds behind the vehicle in front.

In bad conditions, double the safety gap to at least four seconds or even more.

How To Measure

Pedals

Choose an obvious stationary reference point ahead, such as a bridge, a tree or a roadsign. When the vehicle ahead passes the object say to yourself "Only a fool breaks the two-second rule!"

If you reach the object before you finish saying it, You´re too close!

When a vehicle behind is driving too close to you, ease off very gradually and increase the gap between you and the vehicle in front. This will give you more time to react if the driver ahead should slow down or stop suddenly.

Stopping Distance

This is the distance your vehicle travels

  • from the moment you realise you must brake
  • to the moment the vehicle stops

You need to leave enough space between you and the vehicle in front so that you can pull up safely if it slows down or stops suddenly. To do this, you must be able to judge your overall stopping distance.

Practice judging distance when you are walking. Pick out something ahead and see how far away it is. One good stride roughly equals a metre (or yard). Check your estimate and try it out with other objects.

Stopping distance depends on:

  • how fast you are going
  • whether you are traveling on the level, uphill or downhill
  • the weather and the state of the road
  • the condition of your brakes and tyres
  • your ability as a driver & your reaction times when applying the brakes

Stopping distance divides into:

  • thinking distance
  • braking distance

Thinking Distance

This depends on how quickly you react. It takes well over half a second for most people to react and if you are tired, or unwell, this will take even longer.

If you are driving at 20mph, you will travel about 20 feet (6 metres) before your brakes even begin to act

  • at 30mph, 30 feet (9 metres)
  • at 40mph, 40ft (12m) and so on.

Braking Distance

This depends greatly on your speed and the size and weight of your vehicle. It has even more effect on the overall stopping distance.

  • At 20mph, good brakes will stop your vehicle in about 20ft (6m) on a dry road
  • At 40mph (twice the speed), they will take 80ft (24m)- FOUR times the distance

Allow much more time and room to brake in bad weather. Your tyres won´t grip the road surface so well in wet weather or on loose road surfaces.

Calculating The Distances

Overall or Braking distances (OSD) always seem to crop up in the theory test questions and pupils always seem to struggle to calculate them.

Below is a table that may help.

How It Works

  1. OSD (In Feet) = Speed (In Miles) x Speed-Dependant Factor
  2. OSD is also = Thinking Distance + Braking Distance
  3. Thinking Distance (In Feet) = Speed (In Miles Per Hour)

For example, if

  • Speed (In Miles) = 30

Then

  • Speed Factor = 2.5
  • Thinking Distance (In Feet) = 30 (Same as Speed)
  • Multiply Factor x Think Dist (2.5 x 30) = Overall Stop Dist In Feet (75)
  • Subtract Think Dist (30) from Overall Stop Dist (75) = Braking Dist In Feet (45)
Speed
(MPH)
Factor Thinking Dist
(Feet/(metres))
Braking Dist
(Feet/(metres))
Overall Stop Dist
(Feet/(metres))
20 2 20 (6) 20 (6) 40 (12)
30 2.5 30 (9) 45 (14) 75 (23)
40 3 40 (12) 80 (24) 120 (36)
50 3.5 50 (15) 125 (38) 175 (53)
60 4 60 (18) 180 (55) 240 (73)
70 4.5 70 (21) 245 (75) 315 (96)

Don´t worry too much about the factor- just trust that, when used with feet, it works!