A Complete Guide to Calculating Your Thinking, Braking & Overall Stopping Distances
Obviously it is essential for any driver to be able to tell when they need to break or start slowing down, in order to avoid obstacles or stop in time. You also need to be able to understand this issue in order to pass your driving test. A frequent question amongst learner drivers is about how to calculate (and remember) thinking, braking and overall stopping distances. Instead of repeatedly revising and trying to remembering a list of all the different distance and speed combinations (very difficult!), it is much easier – and will make you a better driver IRL in the long run – to be able to just make a quick, yet accurate, calculation for any speed/distance combo. You can learn how to do this by using the simple method outlined below. The method works for calculating the distances in feet – but you can always convert back to metres as necessary.
Overall Stopping Distance (for dry roads)
Your overall Stopping Distance is built from:
- the thinking distance: this is the distance the vehicle has travelled in the time it has taken to initially react to a hazard; and
- the braking distance: this is the distance travelled from the instant the brakes of the vehicle are first applied up until the point when the vehicle comes to a complete stop.
The following table applies a ‘factor’, dependent on the speed you are travelling, which is then multiplied by the speed to give a calculation of the overall Stopping Distance (in ft).
It is surprisingly easy to remember the factors. Just us the following rule: Start at 2 for 20mph, then add 0.5 for every 10mph increase in speed.
You can also, if you wish, calculate each factor by using the following easy formula:
Factor = 1 + [ 0.5 * (speed / 10) ]
Q: If travelling at 50mph, what will be your overall stopping distance ?
A: The relevant ‘Factor’ for 50mph is 3.5. Therefore, the overall stopping distance at 50mph is 50 x 3.5 = 175 ft.
The thinking distance (in feet) is the same as the speed (in mph).
Q: What is your Thinking Distance if you are travelling at 50mph?
A: The Thinking Distance at 50mph is 50 ft.
Your braking distance is the overall Stopping Distance minus the Thinking Distance.
BD = SD – TD
Q: What will be your Braking Distance if travelling at 50mph?
A: Your overall Stopping Distance at 50mph is 175ft (calculated as above). Thinking Distance at 50mph is 50 feet (thinking distance, in ft, is same number as speed, as above). So, your Braking Distance is 175 – 50 = 125 ft.
Converting from feet to metres
If you wish to calculate stopping distances in metres instead of feet, all you need to do is simply do all the calculations in feet, as above, and then convert your final answer into metres. You can do this using the following calculation:
1 foot = 0.3048 metres
Or, to ease calculation, 1ft = 0.3m (approximately). In other words, 1 foot is roughly equal to 3/10 metres.
So, all you need to do us divide the foot distance by 10 and multiply by 3 to get the distance in metres.
Q: What is your overall Stopping Distance, in metres, if travelling at a speed of 40mph?
A: Overall stopping distance at 40mph is (40 x 3) feet = 120 ft. 120ft is approximately equal to 120 * (3/10) metres = (120/10)x3 metres = (12×3) metres = 36 metres.
Stopping Distances on wet or icy roads
Always remember: in wet conditions, the Stopping Distance is DOUBLED.
And on snowy or icy roads, stopping distances can be over 10 times that for normal dry conditions.