Reaction distance: how far you travel before braking
Reaction distance is the distance travelled from recognising a hazard until braking begins. The learning formula is (speed in km/h ÷ 10) × 3: at 50 km/h it gives 15 metres. It assumes a reaction time of 1.08 seconds, roughly one second; real reaction time varies. Braking distance then follows, and both together make stopping distance.
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What happens during the reaction distance?
A hazard appears, you recognise it, decide what to do and begin the braking response. Until braking starts, the car continues to cover ground. The distance during this interval is the reaction distance. Braking distance starts afterwards, once the brakes begin slowing the vehicle.
For a simple calculation, assume the speed stays constant during the reaction interval. At 50 km/h, the car travels about 13.9 metres in exactly one second. During that second, the vehicle has not yet gained any stopping benefit from braking.
One second is a worked-example assumption, not a best value or guaranteed response time for every attentive person. Recognition, the complexity of the situation and the driver's condition can all change the interval. A hazard that is difficult to see may also be recognised later. The legal requirement is to remain attentive and drive to suit the circumstances, not to achieve one fixed reaction-time figure.
The learning formula: speed divided by ten, times three
The familiar learning formula is reaction distance in metres = (speed in km/h ÷ 10) × 3. For example:
- At 50 km/h: (50 ÷ 10) × 3 = 15 metres.
- At 80 km/h: (80 ÷ 10) × 3 = 24 metres.
The physical calculation is distance = speed ÷ 3.6 × reaction time in seconds. The learning formula therefore corresponds to 1.08 seconds, roughly one second. Its small difference from an exact one-second result is not a dependable safety reserve. If recognition or reaction takes longer, the real distance can be much greater.
For the same reaction time, twice the speed produces twice the reaction distance. This differs from the usual braking model: with the same constant deceleration, twice the speed produces four times the braking distance. Keep the two parts separate before adding them to obtain stopping distance.
Reaction distance at common speeds
| Speed | Distance in exactly 1 second | Learning formula, equivalent to 1.08 seconds |
|---|---|---|
| 30 km/h | 8.3 m | 9 m |
| 50 km/h | 13.9 m | 15 m |
| 80 km/h | 22.2 m | 24 m |
| 120 km/h | 33.3 m | 36 m |
The table compares two clearly defined assumptions. At 120 km/h, even an exact one-second reaction interval covers roughly 33 metres before braking starts. The learning formula gives 36 metres because its assumed interval is slightly longer.
If the reaction interval doubles, the distance doubles too: exactly two seconds at 120 km/h means about 66.7 metres. Doubling the learning-model result gives 72 metres, which corresponds to 2.16 seconds. Do not label that 72-metre result as the exact distance in two seconds.
What can delay recognition and reaction?
The actual interval depends on the driver and the situation. These factors matter; none justifies replacing observation with a single universal multiplier:
| Factor | Effect to consider |
|---|---|
| Alcohol | Can impair perception, judgement and response. A fixed alcohol level does not predict one exact reaction time for everyone. If reaction time doubles, reaction distance doubles at the same speed; that is a conditional calculation, not a universal alcohol formula. See Swiss alcohol limits. |
| Fatigue | Can delay perception and decisions. Microsleep can mean seconds with no effective response to the road at all. Stop driving if you are unfit; a larger gap is not a cure. |
| Phones and displays | At 50 km/h, two seconds looking away means about 28 metres travelled without watching the road. Recognition and reaction to a newly noticed hazard may still follow. |
| Medicines and drugs | Some medicines, including sedatives, sleeping tablets and certain painkillers, can impair driving. Follow the medicine information and consult a doctor or pharmacist about driving fitness and interactions. |
| Age and vision | Vision, health and the ability to process a complex scene vary between individuals. Do not infer one person's reaction time from age alone; address difficulties with seeing or responding. |
| Distraction and emotions | Operating navigation or audio equipment, an intense conversation, conflict and time pressure can divert attention from a developing hazard. |
SVG Article 31(2) prohibits driving when alcohol, drugs, medicines or another cause leave you without the necessary ability to drive. Being below a particular alcohol limit does not make impaired driving acceptable.
A phone glance can use up the space you need
Making a call while holding a phone and without a hands-free device is a specific offence carrying a CHF 100 fixed penalty. Other phone use must also be assessed against the duty to pay attention and control the vehicle. Typing, reading a message or another distracting action can lead to ordinary proceedings and potentially licence consequences, depending on what happened. A hands-free system does not make a distracting conversation risk-free.
Consider an example at a steady 50 km/h: two seconds looking away covers about 27.8 metres. If, after looking back, recognising the hazard and starting to brake takes another exact second, you cover another 13.9 metres. That is about 41.7 metres before braking begins. Using the separate 15-metre learning estimate for the final interval would give about 42.8 metres; the assumptions should not be mixed silently.
This is an example, not a prediction for every glance. Set navigation before moving, use a safe place to stop for messages and keep your attention on the changing traffic scene. Looking back at the road does not recover the distance already travelled.
Penalty reference: OBV Annex 1, item 311. The attention duty is set out in VRV Article 3(1).
Following distance: leave room for the unexpected
The Swiss rule is functional: your following distance must allow you to stop in time if the vehicle in front brakes unexpectedly (VRV Article 12(1)). The two-second rule and half the speedometer reading in metres are learning aids, not universal statutory safe distances.
To estimate a time gap, watch the vehicle ahead pass a fixed roadside point and count two seconds before you reach the same point. At 120 km/h, two seconds corresponds to about 66.7 metres. Half of 120 is 60 metres, which at that speed is only 1.8 seconds: the two aids are not mathematically identical.
Leave more room when grip, visibility, traffic or the vehicle requires it. Wet or icy roads mainly affect braking through tyre grip, while spray, rain and fog can also make a hazard harder to recognise. Fatigue or impairment requires addressing fitness to drive, not simply adding a little distance.
Keep the wider picture in view: correct lighting helps you see and be seen, while speed limits set maxima that conditions may require you to stay well below. Neither removes the need for an adequate following distance.
Common theory-test traps
- Reaction distance comes before braking distance. Stopping distance is the sum of both parts.
- The formula (speed ÷ 10) × 3 assumes 1.08 seconds, roughly one second; it is not an exact one-second calculation or a safety guarantee.
- With the same reaction time, twice the speed means twice the reaction distance. Four times the distance applies to the braking model with unchanged deceleration.
- Wetness changes tyre grip, but rain, spray and fog can also delay recognition by reducing visibility. Do not assume weather can only affect the braking part.
- Alcohol, fatigue and distraction can impair responses. There is no universal rule that one particular alcohol level doubles every driver’s reaction time.
Distinctions
- The braking-distance lesson covers the distance after braking begins and the complete stopping-distance table from 30 to 120 km/h.
- Swiss alcohol limits explains the legal thresholds and consequences. Here, alcohol is considered as a possible impairment of perception and response.
- Dipped and main-beam headlights explains visibility at night. A hazard first needs to be recognised before the response can begin; lights do not guarantee a fixed sight distance.
Common questions
How long is a driver’s reaction time?
One second is a learning example, not a guaranteed standard for every attentive driver. Recognition, the complexity of the scene and fitness to drive affect the actual time. Alcohol, fatigue and distraction can impair responses; a particular alcohol level does not create one fixed multiplier for everyone.
How do I calculate reaction distance?
The learning formula is (speed in km/h ÷ 10) × 3. At 50 km/h, that gives 15 metres. The physical formula is speed ÷ 3.6 × reaction time in seconds. The learning formula therefore assumes 1.08 seconds, roughly one second, and provides no guaranteed safety reserve.
How far do I travel in one second at 50 km/h?
About 13.9 metres: 50 ÷ 3.6. The learning formula gives 15 metres because it corresponds to 1.08 seconds rather than exactly one. A real recognition and reaction interval can be longer.
What is the Swiss penalty for using a phone while driving?
Using a phone without a hands-free device while driving carries a CHF 100 fixed penalty under OBV item 311 where the simplified procedure applies. Other distracting phone use is assessed against the attention and vehicle-control duties; depending on the facts, ordinary proceedings and licence consequences can follow.
What is the two-second rule?
It is a following-distance learning aid: watch the vehicle ahead pass a fixed point, then allow at least two seconds before you reach it. Conditions may require more. The legal duty is enough distance to stop in time if the vehicle ahead brakes unexpectedly; two seconds is not a universal guarantee.
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