Whenever I read about the latest bike lights, I see the main criterion most people use to judge them is the brightness, whether it be the number of lumens, lux, glow-worm-power or whatever unit happens to be used by the particular manufacturer.

Of course brightness is a very important specification but I would urge a little caution before playing the numbers game and simply plumping for the most awesome luminary firepower you can get your mitts on. In particular if your primary cycling habitat is the road or cycle path which you share with other road users I would suggest you really don’t need a light with more than a few hundred lumens and anything brighter could be dangerous and actually illegal if not used sensibly.

Regarding the legal aspect, the UK government road vehicle lighting regulations makes for a turgid read but fortunately the guys over at CTC have summarised it very nicely for us. Note in particular that:

Any light fitted to a bicycle must not cause undue dazzle or discomfort to other users of the road.

That’s clear but somewhat subjective. I’ve no idea if anyone’s ever been charged with having a bike light that’s too dazzling but I suspect it’s only a matter of time before it happens. Over the last four or five years I’ve noticed on my daily commute a rising number of cyclists’ lights that are bordering on the uncomfortable, and one chap I met on my route recently was using something akin to a WWII searchlight strapped to his bars. To be fair he was very apologetic and turned it down when I mentioned it but I think he genuinely didn’t realise how dazzling it was.

Apart from the legal aspect, really bright lights need bigger batteries (or have short battery life) and therefore tend to be bulkier and heavier. Of course they are more expensive too…

Most of the brightest bike lights are designed for off road cycling. For off-road you do want a nice bright light to light up the trail and also to see up to head height in case you’re riding through a wooded area. On roads though it’s a different matter. Some lights have a beam pattern designed specifically for road use. For example a letterbox or wedge shaped beam pattern can provide maximum brightness on the road and don’t fire so much light into the sky or directly into the eyes of approaching motorists.

So how many lumens is enough?

This is a question I’ve been asked a few times. I find my 600 lumen Exposure Strada light gives me just the right amount of light to ride with confidence at speed on my route which includes pitch dark country lanes. You can get away with considerably less light but you need to ride more slowly. Actually, the Strada does cause dazzle on the high setting but it’s very easy to dip the beam.

The 450 lumen Lezyne Super Drive also provides plenty enough light for dark country lanes and is significantly cheaper than the Strada.

If you really want to use a seriously bright light on the road, I would suggest making sure the light you buy has a number of brightness modes and that it’s easy to switch between them while riding. Spend some time carefully setting up the light. Pointing it slightly downwards can help to avoid dazzle.

Above all, use common sense. If you’re finding approaching car drivers stop, flash their lights, shout abuse or swerve off the road into a ditch when you are approaching, it probably means your light is too dazzling and should be switched to a lower setting. It’s important to see and be seen but a bit of consideration and courtesy to our fellow road users goes a long way.