When to use hazard warning lights
According to the Highway Code, you should only use hazard lights when your vehicle is stationary and causing an obstruction. For example, if you’ve broken down on the hard shoulder.
The exception is if you’re on a motorway or unrestricted dual carriageway, when you can use your hazards, briefly, to warn people behind you of an obstruction or traffic jam up ahead.
Why? Well, hazard lights make use of your indicator bulbs. So if you use them while in motion, it could prevent you from signalling effectively.