A hazard is any situation that involves you being in some risk or danger. Hazards may cause you to slow down or change course.
When approaching a hazard, you should use your mirrors and be prepared to change speed and/or direction.
Hazards include
Always check your mirrors in good time, and before you change direction, to decide
Keep up to date with the position and speed of traffic behind you.
Good drivers always know as much about the conditions behind as they know about the situation ahead.
Traffic positions change rapidly on some roads. Frequent glances in mirrors keep you up to date with what’s behind. How frequently you do this depends on road and traffic conditions.
When driving on motorways or dual carriageways, check your mirrors earlier than you would on ordinary roads.
Higher speeds are more difficult to judge and situations can develop more quickly.