Daytime running light
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.
Since 2011, DLRs are mandatory for all new cars and small delivery vans in the EU. Trucks and buses followed in August 2012. Vehicles produced before don't have to be retrofitted.