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.