When you’re cycling at night, the law and common sense demand lights. The front must be white, the rear red, and both must be mounted to the bike. You’re free to use additional lights – on you helmet, for example, or on a rear rack as well as the seatpost – so long as they’re the right colour. Flashing lights are tolerated. While there’s more to lighting regulations than that, these points generally satisfy the police. Legal is one thing; whether your lights are practical is another matter. Here’s a rough guide to battery lighting.
Urban only: 50+ lumens
When you’re cycling under streetlights, the primary job of your lights is to show other road users where you are. Flashing lights are effective as they draw the eye, but to be road legal by themselves flashers must flash one to four times per second and emit at least four candela (about 50 lumens). That’s a sensible minimum whether your lights are flashing or steady. Quick-release lights on the handlebar and seatpost are the most common. Bolt-on lamps that can be left on the bike provide get-on-and-go convenience for urban cycling but are rare, aside from dynamo setups and lamps that fit to rear racks.
Rural or mixed: 200+ lumens
On unlit rural roads you need a front lamp that shines further ahead than your stopping distance, which is about 20 metres on a dry road at 20mph. Even with a well focused beam, you’ll likely need at least 200 lumens. If you use a much more powerful headlight, treat its higher setting(s) like full beam on a car and switch to a lower mode to avoid dazzling oncoming road users. Whatever front lamp you get, make sure it’s rechargeable or you’ll spend loads on batteries. For the rear, the same kind of light you’d use for urban use is fine, except that a steady or pulsing mode is better than flashing as it’s harder for other road users to track your position in the dark if your light blinks on and off.
Off-road: 1,000+ lumens
On off-road trails, you need more light to see clearly what you’re riding over, plus better peripheral vision to avoid tree branches etc. Two front lights is ideal, at least one of them 1,000 lumens or more. Mount one lamp on your helmet and one on your handlebar. If you use just one, make it the helmet light so you can always see where you look. A rear light can be mounted to the back of your helmet, seatpost, or saddle rails. Bear in mind that it can become obscured by mud flung up by the rear wheel. Unless your route is entirely off-road, you’ll also need lamps on the bike to satisfy road regulations; small flashers front and rear may be sufficient.
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As well as lights and reflectors, cycle commuting at night requires some adjustments to the way that you ride.