Daytime running lights are designed to come on automatically when the engine is started
You've probably noticed those bright LED 'eyebrows' on newer cars, but why are so many cars being fitted with them, and what are they for?
European legislation adopted in 2008 required dedicated daytime running lights (DRL) to be fitted to all new 'types' of passenger cars and small delivery vans since February 2011. Trucks and buses followed from August 2012.
Daytime running lights are designed to come on automatically when the engine is started - all other lights should remain off.
Note: This does not mean that every new car first registered after February 2011 will have DRL fitted. The requirement only applies to models that go through the European whole vehicle type approval process after that date i.e. new or substantially facelifted models.
There is no requirement to retro-fit DRLs to existing cars and no Europe-wide requirement for drivers of cars without daytime running lights to drive with headlights on during the day.
If you're driving abroad, check our touring tips for local rules.
Daytime running lights must be bright enough that they can be seen clearly in daylight and as a result are too bright to be used at night time when they would cause dazzle. Daytime running lights should therefore go off automatically when headlights are switched on.
Daytime running lights don't have to be separate lights - some car manufacturers combine them with the front position lamps (side lights) in which case the daytime running lights will dim when the headlights are turned on.
If daytime running lights are located very close to indicator lights then the DRL on the appropriate side of the vehicle must turn off while the indicator is operating to avoid masking its signal.
The performance requirements for Daytime running lights are defined in UN ECE Regulation 87: Daytime running lights
The operating requirements for Daytime running lights are defined in UN ECE Regulation 48: Installation of lighting and light-signalling devices.
The requirement to fit daytime running lights applies to new cars only. There is no requirement to retrofit daytime running lights but kits are available if you wish to do so. Lights approved to the appropriate EU legislation will have an approval mark on the lamp which includes the letters 'RL'.
Retrofitted daytime running lights should be installed so that they come on with the engine and go off when headlights are turned on. Manually operated daytime running lights must be turned off at night to avoid dazzling other road users.
The Volvo 240 was the first car in Britain that ran with lights on all the time the engine was running. Although they looked like sidelights they were in-fact separate 21 watt bulbs located alongside the standard 5 watt sidelight bulbs. The brighter bulbs were lit all the time unless the driver turned on the sidelights.
In the mid-1980s the UK planned to introduce 'dim-dip' lighting for use in urban areas at night, rather than for daytime use but plans were scuppered when common European standards couldn't be agreed.
Dim-dip switched on headlights at reduced brightness when the ignition and side lights were on. Volvo and others adopted dim-dip in place of 21 watt side lights for daytime running at this time.
A lot of discussion followed about whether cars across Europe should have their headlights on all the time – many countries introduced local rules while the European commission considered legislation and possible technical standards.
By 2006 drivers in 12 countries had to drive with their headlights on all year round including Sweden since 1977, Iceland, Latvia, Macedonia and Norway since around 1980, Denmark since 1990 and Romania, Slovenia and parts of Portugal since 1998.
Lithuania, Slovakia and the Czech Republic required daytime use of headlights in the winter only while Hungary and Italy required daytime running lights outside built up areas.
The UK never introduced a rule requiring daytime use of headlights.
Those in favour of daytime running lights claimed they significantly reduce road deaths and serious injuries, while those against objected to the constant glare of headlights and voiced concern that motorcycle riders may become less conspicuous. There was some concern about increased fuel costs too.
In 2006 the European Commission published the results of research into the effectiveness, costs and benefits of introducing Daytime Running Lights (DRL).
A European Commission study in 2006 suggested that a substantial number of casualties could be prevented across the EU with a positive benefit-to-cost ratio when the costs of fitting lamps and the environmental cost of running them was taken into account.
A later UK Department for Transport (DfT) study confirmed the Commission's findings that there would be a net reduction in accidents, but cast doubts about whether the benefit would outweigh the costs.
The UK study also concluded that dedicated Daytime Running Lights could improve the visibility of cars in dim light without reducing the conspicuity of motorcyclists.
Light Emitting Diodes (LED) only consume a fraction of the electricity taken by a normal headlight - they're being used increasingly for other lights on vehicles for the same reason.
Use of dedicated daytime running lights instead of driving with headlights or sidelights also means that tail lights and instrument lights are not illuminated during the day.
The alternator is driven by the engine and spins all the time but it doesn't always consume the same amount of power from the engine. When the electrical load on the alternator increases more power is required to turn it and so fuel consumption is increased.
While headlamps consume 110 watts of power, dedicated daytime running lights using LEDs might consume only 5-10 watts so putting negligible load on the alternator.
(updated 10 October 2016)