If you are driving down a steep downgrade, you should

If you keep applying the brakes to control your speed on a steep descent, the brakes may fade (lose their effectiveness). Instead, release the gas pedal and shift into a lower gear (even with an automatic transmission). This will generate a braking effect to slow the vehicle, known as engine braking. The lower the gear, the greater the engine braking effect. Apply the brakes only when engine braking is insufficient or you want to stop.