How to Calculate Thinking, Braking & Overall Stopping Distances
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. This method works for calculating the distances in feet. You can convert back to metres afterwards if necessary.
The factors are easy to remember - just start at 2 for 20mph and add 0.5 for each 10mph increase in speed. Alternatively, you can calculate each factor by using the following simple formula:
factor = 1 + [ 0.5 * (speed / 10) ]
Example
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 Distance
The thinking distance (in feet) is the same as the speed (in mph).
Example
Question: What is the thinking distance at 50mph?
Answer: Thinking distance at 50mph is 50 feet.
Braking Distance
The braking distance is the overall stopping distance minus the thinking distance.
Example
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.
Converting feet to metres
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.
Example
Question: What is the overall stopping distance in metres distance 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.
Stopping distances on wet or icy roads
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.