The **reaction time** is the period it takes for a driver to respond to a hazard after detecting it, such as seeing a red traffic light or an obstacle on the road. This time is crucial because it determines how long it takes for the driver to start acting, like pressing the brake.

- **Reaction time**: It is the time that passes from when the driver sees or hears something until they react. Under normal conditions, this time is usually between half a second and one second.
- **Factors affecting reaction time**: The physical and psychological state of the driver can influence this time. For example, being tired, sleepy, consuming alcohol or drugs, and not paying attention can increase the reaction time.

- **Difference with other times**: It should not be confused with the time it takes for the vehicle to come to a complete stop, which includes both the reaction time and the braking time.

Therefore, the reaction time is specifically the time it takes for the driver to start reacting to a hazard, which makes the first response the correct one.