Don't spend too much time looking at a single mirror because:

Question

Check you mirrors frequently while driving, but check them quickly. Staring at one mirror for too long can cause you to drive a long distance without knowing what is ahead.

Accepted Answer

It can result in you driving a long distance without looking at the road.

Answer

Conditions can change quickly.

Answer

Looking at mirrors can make you drowsy.

Answer

The mirror may not provide an accurate view of the road.