What this calculator does
Before a driver even starts braking, the vehicle keeps travelling at full speed for the length of their reaction time, the gap between perceiving a hazard and pressing the brake pedal. That distance is simply speed multiplied by reaction time, and it is entirely separate from the braking distance that follows once the brakes actually engage.
Published average driving reaction time is commonly cited around 1.5 seconds, though it varies with alertness, distraction, age and the complexity of the hazard, which is why this calculator leaves it as an editable input rather than a fixed assumption. Even a small change in reaction time has a large effect on stopping distance at speed, because that portion of the journey happens before any braking force is applied at all.
The formula
Convert speed to metres per second, then multiply by reaction time in seconds to get the distance covered before braking begins. This is purely the reaction-time component of stopping distance; it does not include the separate distance the vehicle travels while the brakes are slowing it down.
| Term | Meaning |
|---|---|
| Reaction distance | Distance travelled at constant speed during the reaction time, before braking starts. |
| Speed | The vehicle's speed at the moment the hazard is perceived. |
| Reaction time | The time between perceiving the hazard and physically applying the brakes. |
The inputs explained
| Field | What to enter |
|---|---|
| Speed (km/h) | The speed the vehicle is travelling at when the hazard first appears. |
| Reaction time (s) | The driver's reaction time. A commonly cited average is around 1.5 seconds, but this varies by person and circumstance, so adjust it to suit. |
When to use it
Understanding why speed matters so much
Reaction distance rises in direct proportion to speed, so doubling speed doubles the distance covered before braking even starts, on top of the separate, larger increase in braking distance itself once the brakes are applied.
Teaching new drivers about following distance
Reaction distance alone, before any braking, illustrates why a following gap needs to allow for the fact that nothing is slowing the vehicle down during that first second or so.
Accounting for impaired or distracted reaction time
Fatigue, distraction or alcohol can roughly double typical reaction time; recalculating with a longer reaction time shows how much extra distance that adds before braking even begins.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How reaction distance changes with speed at a 1.5 second reaction time
A fixed 1.5 second reaction time, across a range of speeds.
| Speed | Reaction distance |
|---|---|
| 30 km/h | 12.50 m |
| 50 km/h | 20.83 m |
| 60 km/h | 25.00 m |
| 80 km/h | 33.33 m |
| 100 km/h | 41.67 m |
| 120 km/h | 50.00 m |
Questions
Is 1.5 seconds the correct reaction time to use?
It is a commonly cited average for an alert driver reacting to an expected hazard, but actual reaction time varies with age, fatigue, distraction and how unexpected the hazard is. Treat it as a reasonable default rather than a fixed figure, and adjust it if you have a specific scenario in mind.
Does this include the distance travelled while braking?
No. This calculator covers only the reaction-time portion, when the vehicle is still travelling at full speed because the brakes have not been applied yet. Total stopping distance also includes the separate braking distance once the brakes engage, which depends on deceleration rate rather than reaction time.
Why does reaction distance matter if braking distance is usually longer?
At lower speeds, braking distance can be short enough that the reaction-time distance is actually the larger share of the total stopping distance, which is often underestimated because braking itself feels like the main event.
How much does distraction increase reaction time?
Studies on distracted driving, such as using a mobile phone, commonly report reaction times roughly double an alert baseline. Doubling reaction time in this calculator directly doubles the distance travelled before braking begins.
For the deceleration that follows once the brakes are actually applied, see the acceleration and deceleration figures in the inclined plane calculator and related motion calculators.