Stopping Distance in Japan: Reaction Distance + Braking Distance

What is stopping distance equal to?

Stopping distance is the reaction distance plus the braking distance. The Rules of the Road (交通の方法に関する教則 kōtsū no hōhō ni kansuru kyōsoku, the official study text behind the written test) puts it like this: a car cannot stop instantly, so stopping needs the distance the car travels between the driver sensing danger and the brakes actually starting to work — the reaction or free-running distance, 空走距離 — added to the distance from the brakes taking hold until the car stops, the braking distance, 制動距離.

The practical consequence is the part people skip: pressing the brake is not the moment the car begins to slow. There is a gap first, and that gap is measured in metres, not in feelings. The test states the identity plainly — see stopping distance = reaction + braking — and expects you to drive at a speed where the total still fits inside the distance you can actually see.

What makes the reaction distance longer?

Anything that slows down how fast you notice and decide — fatigue above all. The Rules of the Road state that when a driver is tired it takes longer to perceive and judge a hazard, so the reaction distance becomes longer.

This is a favourite trap, because the test writes it backwards: 'when a driver is tired they notice danger sooner, so the reaction distance becomes shorter'. That is false. Keep the division of labour in mind and these questions stop being confusing: the reaction distance depends on the driver, while the braking distance depends on the car, the tyres and the road. That is exactly why the two are counted separately instead of being rolled into one number.

What makes the braking distance longer?

A wet road, worn tyres, a heavy load, and a downhill gradient. On wet asphalt the friction between tyre and road drops, so the car needs more distance after the brakes bite (wet asphalt), and the same is true when you are carrying a heavy load.

What the test really tests is the combination. A wet surface together with worn tyres can stretch the stopping distance to about twice what it would be on a dry road with tyres in good condition (about twice). So rain makes the stopping distance longer, never shorter — a statement you will meet in the reversed form. On a downhill slope the car also gains speed by itself, which is why the rule there is to widen the gap to the vehicle ahead rather than simply to brake later.

If my speed doubles, what happens? (the ×4 rule)

Everything dangerous becomes four times bigger, not twice. The Rules of the Road say it explicitly: if speed doubles, the braking distance, the force trying to slide or tip the car over in a curve, and the impact force in a crash all become four times greater, because each is proportional to the square of the speed.

So the statement 'if your speed doubles, the braking distance and the impact force also double' is false — the factor is 4. The same square law governs centrifugal force in a bend, and there is a second variable attached to it: centrifugal force grows as the radius of the curve gets smaller, not larger. Tight bend plus high speed is the worst possible pairing, and the test likes to invert one half of it.

How hard is the impact at 60 km/h?

Hitting a concrete wall at 60 km/h delivers roughly the impact of falling from about 14 metres — the height of a five-storey building. That figure is given in the Rules of the Road, and the written test quotes it almost word for word (14 m, about 5 floors).

Impact force is not decided by speed alone. It grows with both speed and the weight of the vehicle, and it is larger when the impact happens over a shorter time — which is why striking something hard and unyielding is worse than striking something that deforms. The statement 'impact force depends only on speed and has nothing to do with the vehicle's weight' is therefore false. Two variables, not one: that pattern alone answers several questions in this topic.

How does this turn into a following distance?

By law you must keep enough distance to avoid a rear-end collision even if the vehicle in front stops suddenly (Road Traffic Act Art. 26). The duty is written for 'vehicles etc.', so it is not limited to cars — mopeds and light vehicles carry it too. Note that no fixed number of metres is set for ordinary roads: the required distance is whatever the weather, road surface, tyres and load make necessary, which is why 'you must always keep at least 50 m' is not the rule.

For expressways the Rules of the Road do give figures. On a dry surface with new tyres you need about 100 m at 100 km/h and about 80 m at 80 km/h (100 m at 100 km/h) — conveniently, about the same number of metres as your speed in km/h. With a wet road and worn tyres you may need about twice that, not half. And one perception trap worth remembering: in a large or medium-sized truck the high driver's seat makes the gap feel longer than it really is, so the actual gap tends to become too short.

FAQ

Stopping distance is equal to reaction distance plus what?

Plus the braking distance. Stopping distance = reaction (free-running) distance + braking distance. The reaction distance is covered between sensing danger and the brakes taking hold; the braking distance runs from there until the car stops.

What is the free-running or reaction distance?

It is the distance the car travels from the moment the driver senses danger until the brakes actually start to work. In Japanese it is called kūsō-kyori (空走距離). It gets longer when the driver is tired, because perceiving and judging the hazard takes more time.

When the speed is doubled, what is the collision force factor?

Four. Braking distance, the force that tries to slide or tip the car in a curve, and the impact force in a collision are all proportional to the square of speed, so doubling your speed multiplies them by four, not by two.

Does the car start slowing the instant I press the brake?

No. There is a gap before the brakes take hold, and the car keeps travelling during it — that is the reaction distance. Only after the brakes bite does the braking distance begin. The two added together are the stopping distance.

How much longer is the stopping distance on a wet road with worn tyres?

About twice as long as on a dry road with tyres in good condition. Wet asphalt reduces friction and worn tyres reduce grip, and the two effects combine.

What following distance do I need at 100 km/h?

About 100 metres on a dry surface with new tyres, and about 80 metres at 80 km/h. On a wet road with worn tyres you may need roughly double. On ordinary roads no fixed figure is set — the law requires whatever distance lets you avoid a rear-end collision if the vehicle ahead stops suddenly.