Daytime running lights (DRLs) come on automatically whenever a vehicle's engine is started. They substantially increase the visibility of cars and other vehicles.
In many countries, headlights are compulsory only in the dark. But DRLs are designed to be on during the day too. Their job isn't to help the driver see the road but to help other road users see the vehicle. Daytime running lights consume just 25-30% of the energy needed to power a conventional dipped headlight.
From 2011 onwards, DLRs will be mandatory for all new cars and small delivery vans in the EU. Trucks and buses will follow in August 2012. Existing vehicles will not have to be retrofitted. Currently, 17 EU countries already have some form of DRL legislation.