How to use the reaction distance calculator
The formula
Reaction distance = speed (m/s) × reaction time (s)
Speed in mph is converted to metres per second (mph × 0.44704). At 30 mph that is 13.4 m/s — so every tenth of a second of reaction time costs another 1.34 metres of road.
Why it matters
Reaction distance is the part of stopping that no brakes can fix: it depends entirely on the driver noticing and responding. It is why road-safety education focuses so heavily on attention — doubling reaction time from 1 to 2 seconds at 30 mph adds a full 13 metres, about three car lengths, before braking even begins. For the complete picture, pair this with the Stopping Distance Calculator.
Example
At 30 mph with a 1-second reaction time, a car travels 13.4 metres (44 feet) — about 3 car lengths — before the driver touches the brake. At 2 seconds, that doubles to 26.8 metres.
Frequently asked questions
What is reaction distance?
The distance a vehicle travels between the driver spotting a hazard and physically pressing the brake. At 30 mph a car covers about 13.4 metres every second, so even a one-second reaction means over 13 metres travelled with no slowing at all.
How long is one car length?
This visualiser uses 4.4 metres (about 14.4 feet), the length of an average family car. Real cars vary, so car lengths are an illustration, not a measurement.
What reaction time should I use?
The UK Highway Code's thinking distances assume roughly 0.67 seconds. An alert driver in tests often reacts in about 0.5–1 second; tiredness, age, medication, and phone distraction can push it to 2 seconds or more. Try 0.7 s for alert and 2.0 s for distracted to see the difference.
Is reaction distance the same as stopping distance?
No. Reaction (thinking) distance is only the first part. Total stopping distance = reaction distance + braking distance. Use the Stopping Distance Calculator for the full figure.
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