To effectively reduce the speed of a vehicle, it is important to use **both the brakes and the engine**.

- **Brakes**: When you press the brake, the vehicle begins to stop. The distance it travels from the moment the brake is pressed until it comes to a complete stop is called the **braking distance**. This distance can vary depending on the speed, the load of the vehicle, technical conditions (such as the state of the brakes and tires), the weather, and road conditions.

- **Engine**: By lifting your foot off the accelerator, the engine helps to gradually reduce the vehicle's speed. This is part of efficient driving, which involves letting the vehicle roll without accelerating and using the brakes gently to decrease speed gradually.

Therefore, combining the use of the brakes and the engine is the safest and most efficient way to reduce the speed of a vehicle.