A common question asked is how to remember thinking, braking and overall stopping distances. Rather than remembering a list of the distances, you can calculate them instead using the method detailed below. The calculations are not difficult - just involving some elementary arithmetic. **The method given below works for calculating the distances in feet**. You can convert back to metres afterwards if necessary.

The overall stopping distance is built from

• **the thinking distance**, i.e. the distance the vehicle has travelled in the time taken to react to a hazard; and

** • the braking distance**, i.e. the distance travelled from the moment the brakes of the vehicle are applied to the point when the vehicle comes to a complete stop.

The table below applies a 'factor', dependent on the speed you are travelling, which is multiplied with the speed to calculate the overall stopping distance in feet.

SPEED | FACTOR | OVERALL STOPPING DISTANCE (ft)
(speed x factor) |
---|---|---|

20 | 2 | 40 |

30 | 2.5 | 75 |

40 | 3 | 120 |

50 | 3.5 | 175 |

60 | 4 | 240 |

70 | 4.5 | 315 |

The factors are easy to remember - just start at 2 for 20mph and add 0.5 for each 10mph increase in speed. Alternatively, for those more mathematically inclined, you can calculate each factor by using the following simple formula:

**factor = [(speed ÷ 10) ÷ 2] + 1**

So, for example, the factor for 40mph is [(40 ÷ 10) ÷ 2] + 1 = [4 ÷ 2] + 1 = 2 + 1 = 3.

**Question: What is the overall stopping distance at 50mph? **

**Answer: Factor for 50mph is 3.5 and so overall stopping distance at 50mph is 50 x 3.5 = 175 feet.**

Thinking distances are easy! The thinking distance (in feet) is the same as the speed (in mph).

**Question: What is the thinking distance at 50mph?**

**Answer: Thinking distance at 50mph is 50 feet.**

The braking distance is the overall stopping distance minus the thinking distance.

**Question: What is the braking distance at 50mph?**

**Answer: Overall stopping distance at 50mph is 175 feet (see above). Thinking distance at 50mph is 50 feet (see above). So braking distance is 175 - 50 = 125 feet.**

If you want to calculate stopping distances in metres instead of feet, simply do the calculations in feet and then convert the final answer back to metres:

1 foot = 0.3048 metres, or, to ease calculation, 1 foot is roughly equal to 3/10 metres, i.e. divide the distance by 10 and multiply by 3.

**Question: What is the overall stopping distance in metres at 40mph?**

**Answer: Overall stopping distance at 40mph is 40 x 3 feet = 120 feet. 120 feet is approximately equal to 120 * (3/10) metres = (120 ÷ 10)*3 metres = 12*3 metres = 36 metres. **

Remember in wet conditions stopping distances are **doubled**. On snow or icy roads stopping distances can be **over 10 times** that of normal dry conditions.