Perception and reaction are two separate delays
A lab reaction test measures a response to a signal you were already watching for. Driving does not work like that. First something has to enter your awareness — a brake light, a child at the kerb, a car drifting across a line. Then you have to recognise it as a hazard. Only then does the reaction you can measure on a screen begin.
Together these take roughly 0.7 to 1.5 seconds for an alert driver who is looking in the right direction. Road design standards use a more conservative 2.5 seconds, because designers have to account for drivers who are not alert and not looking in the right direction — which, on any given road, is a lot of them.
What that costs in metres
| Speed | Metres per second | Distance covered in 1.0 s |
|---|---|---|
| 30 km/h (19 mph) | 8.3 m/s | 8.3 m |
| 50 km/h (31 mph) | 13.9 m/s | 13.9 m |
| 70 km/h (43 mph) | 19.4 m/s | 19.4 m |
| 100 km/h (62 mph) | 27.8 m/s | 27.8 m |
| 130 km/h (81 mph) | 36.1 m/s | 36.1 m |
Reaction distance scales linearly with speed. Braking distance scales with the square of speed, which is why a small increase in speed produces a large increase in total stopping distance.
What makes it worse
- Phones. Reading a message takes your eyes off the road for several seconds. At 100 km/h, four seconds is more than a hundred metres travelled blind. No reaction time can recover that.
- Alcohol. Slows reaction and, more dangerously, degrades judgement about whether you are slowed.
- Fatigue. Produces microsleeps of several seconds during which you are not reacting at all.
- Unexpected hazards. Recognising something you did not expect takes markedly longer than responding to something you did.
- Night and poor weather. Less contrast means the hazard registers later, before reaction even starts.
The two-second rule does the work for you
Pick a fixed object ahead, and count two seconds between the car in front passing it and you passing it. That gap covers a normal perception-reaction time with margin, at any speed, without arithmetic. Double it in rain, and give yourself more again at night or behind a vehicle you cannot see past.
Following distance is the single most effective thing you control. It is worth more than any difference in reaction time between two healthy drivers, and unlike reaction time you can change it instantly.
Testing yourself is a useful demonstration, not a licence check
Running the reaction test is a good way to feel how long a quarter of a second is, and a striking way to show a learner driver what happens to that number after a bad night's sleep. It is not an assessment of fitness to drive. Real driving reaction depends on attention, scanning and hazard recognition, none of which a browser can measure.