Explanation

"Advance warning triangles fold flat and don’t take up much room. Use one to warn other road users if your vehicle has broken down or if there has been an incident. Place it at least 45 metres (147 feet) behind your vehicle (or the incident), on the same side of the road or verge. Place it further back if the scene is hidden by, for example, a bend, hill or dip in the road. Don’t use warning triangles on motorways."