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After dark, yes. A thousand times yes.
In the UK, bike lights after dark are not only extremely sensible, they’re a legal requirement according to the Road Vehicles Lighting Regulations.
Meanwhile, Rule 60 of the Highway Code states:
“At night your cycle MUST have white front and red rear lights lit. […]Flashing lights are permitted but it is recommended that cyclists who are riding in areas without street lighting use a steady front lamp.”
The lights also need to be fixed to the bike. That means that lights that fit onto your helmet on the back of a rucsac, while helping you be more visible, do not meet the legal requirements for bike lights.
Are bike lights necessary during the day?
Legally speaking, no. Light manufacturers increasingly have ‘day time modes’ involving a bright strobe effect, but whether or not a cyclist uses them is a matter of personal preference. Some people feel safer, and others feel it is unnecessary.
Street lamps are not enough
Some people (we’ll call them “night ninjas”) assume that if they’re cycling without lights but under street lamps, drivers will be able to see them anyway. That just isn’t the case. In fact it can be extremely difficult to spot a slow moving dark shape on the road, and cycling without lights is dangerous.
Bright front lights also help you see
It’s great to be able to see exactly where you’re going. It may seem obvious, but a bright front light – say, of 200 lumens in the city or 600 off road – will allow you to spot pot holes, broken glass, and maybe even an abandoned £20 note.
What’s the minimum light output I need on my bike?
Front Light: A flashing light needs to be at least ‘4 candelas’, which equals roughly 50 lumens, which is fairly modest. A bike light that can put out a steady light should be marked as conforming to BS6102/3 or an equivalent EC standard (read more about this on Cycling UK). Lights from reputable brands such as Cateye, Lezyne, Exposure, and Moon meet this standard.
Rear Light: Rear lights in general emit fewer lumens than front lights, but if they are flashing then they must also emit at least 4 candelas. Again, lights from reputable brands such as Cateye, Lezyne, Exposure, and Moon meet this standard.
Can a bike light be too bright?
Legally speaking, it’s somewhat open to interpretation. The Road Vehicles Lighting Regulations say it is an offence for vehicles (any vehicles) to cause “undue dazzle or discomfort to other persons using the road”.
Morally speaking, it’s nice to be nice, and it’s also nice to not cause other people to have accidents. Nobody likes to be blinded by someone else’s lights. When riding on shared paths in particular, dip your lights down instead of pointing 1,000 lumens straight at people’s faces (including other cyclists.)
If you want to geek out on the brightness of bicycle lights, here’s a good article by Chris Juden called ‘Dazzling Bike Lights’.