Reaction, Braking and Stopping Distance in Japan's Driving Test

Reaction distance vs braking distance: comparison table

The National Police Agency's Rules of the Road (交通の方法に関する教則) say that to stop, a car needs the distance it travels "from when the driver senses danger and presses the brake until the brake actually starts to work (free running distance)" plus the distance "from when the brake starts to work until the car stops (braking distance)"; together these make the stopping distance.

So the three distances are told apart by where they start and end. Remember that the reaction distance depends on the driver's time, and the braking distance on the car and the road surface. That also answers the "what makes which distance longer" questions.

Stopping distance is the sum of the two

Stopping distance covers everything from the moment danger is sensed until the car is fully stopped. The most common trick in the knowledge test (知識確認) is swapping the starting point. "From when the brakes start to work until the car stops" is the braking distance; "from sensing danger until the brakes work" is the reaction distance. Any statement that calls only one of them the stopping distance is false.

The reaction distance starts when danger is sensed, not when the brake pedal is pressed. While you notice the danger, move your foot to the brake and press it, the car keeps going at full speed.

Real test question: "Stopping distance is the distance from when the brakes begin to work until the vehicle stops." → Answer: FALSE (×). That is the braking distance.

• Reaction distance: sense danger → brakes start to work
• Braking distance: brakes start to work → car stops
• Stopping distance: sense danger → car stops (reaction + braking)

Speed: braking distance grows with the square

The Rules of the Road state that "braking distance, centrifugal force and impact force all increase in proportion to the square of the speed": if speed doubles, braking distance becomes four times as long, not twice. Because it is squared, three times the speed means nine times the distance.

If reaction time stays the same, the distance covered during it grows in direct proportion to speed. So at higher speed both parts get longer, the braking part dramatically, and total stopping distance rises quickly.

As an example of impact force, the Rules of the Road say that hitting a concrete wall at 60 km/h is about the same as falling from a height of about 14 m (roughly the fifth floor of a building).

Real test question: "If your speed doubles, your braking distance also roughly doubles." → Answer: FALSE (×). It is about four times.

Which condition lengthens which distance

Questions often swap the cause and the distance it affects. Split them: the driver's condition affects reaction distance; road surface, tyres and load affect braking distance.

According to the Rules of the Road, on a wet road with worn tyres the stopping distance can be about twice as long as on a dry road with good tyres. Going downhill also builds speed and lengthens the stopping distance, so leave more room.

Real test question: "When the road is wet with rain and the tyres are worn, both the reaction distance and the braking distance become at least twice as long." → Answer: FALSE (×). Road and tyre condition affect the braking distance; the reaction distance depends on the driver's reaction time.

Following distance: the numbers in the Rules of the Road

Drivers must allow for stopping distance and keep enough gap that they will not hit the car ahead if it stops suddenly (Road Traffic Act Art. 26; Rules of the Road). For expressways, the Rules of the Road say that on a dry road with new tyres you need about 100 m at 100 km/h and about 80 m at 80 km/h, and that on a wet road with worn tyres about twice that may be needed.

On rainy days, the Rules of the Road ask you to drive slower than on sunny days and keep a generous gap. They also warn that drivers of large, medium and semi-medium vehicles sit high, so the gap looks longer than it is and tends to become too short.

Trick questions summary and practice

When a question contains 空走 (kūsō), 制動 (seidō) or 停止 (teishi), first check the start and end points, then whether the cause is on the driver's side or the road's side. Knowing the Japanese terms, their readings and meanings will keep you calm when the question appears in Japanese.

It helps to work through all the questions on these three distances in a mock test you can read in English.

• "Reaction distance runs from pressing the brake until it works" → FALSE (from sensing danger)
• "Stopping distance equals braking distance" → FALSE (reaction + braking)
• "Fatigue makes the braking distance longer" → FALSE (reaction distance)
• "Braking distance is longer on a road wet with rain" → TRUE
• "At twice the speed, braking distance is about four times" → TRUE

Practice questions using this term

• If your speed doubles, your braking distance also roughly doubles.
• Stopping distance means the distance the car travels from the moment the brakes start to work until it comes to a stop.
• On a road wet with rain and with worn tires, both the free running (reaction) distance and the braking distance become at least twice as long.
• On rainy days, visibility is poor and the braking distance is longer, so it is good practice to keep about twice the following distance you would keep on a clear day.

FAQ

Related guides

• Japan Speed Limits: 60 km/h, 30 km/h Residential Roads, Mopeds 30
• Japan Expressway Rules: Max 100, Min 50 km/h, Who Can't Enter
• Shuki-obi vs Sake-yoi: Japan's Two Drink-Driving Offenses
• Japan Statutory vs Posted Speed Limits (法定速度 vs 規制速度)
• How to Study for Japan's License Conversion Knowledge Test

Sources (in Japanese)

• 警察庁「交通の方法に関する教則」（令和6年11月13日現在・全文PDF。第4章第4節「停止距離と車間距離」、自動車の運転に関する自然の法則、高速道路の車間距離）
• e-Gov法令検索「道路交通法」（第26条 車間距離の保持）