A rear light is a legal requirement when cycling at night, and using one that's brighter than the legal minimum seems like a sensible way of helping drivers see you — or at least defanging 'but I didn't see them' excuses. The best bike rear lights have long run-times, can be seen from a good distance, and are sufficiently tough and waterproof to fend of day-to-day abuse.
Cycling rear lights universally use one or more red LEDs to generate their light
Rechargeable rear lights that take power from a USB source are popular, but battery-powered lights are still available
Rear bike lights are increasingly intended as day-time safety lights too
A flashing red light says 'bike' to most drivers; we recommend using a constant light as well so your position can be easily followed
Need a front light too? Check out our guide to cycling front lights
A rear light has the proverbial one job: to get you seen, unlike a front light that has to be visible enough to stop inattentive drivers mowing you down while allowing you to see where you're going. Depending on where and how you ride, your priorities regarding brightness, flashing modes and battery life will be different.
The Road Vehicle Lighting Regulations say:
One is required, to show a red light, positioned centrally or offside, between 350mm and 1500mm from the ground, at or near the rear, aligned towards, and visible from, behind. If capable of emitting a steady light, it must be marked as conforming to BS3648, or BS6102/3, or an equivalent EC standard. If capable of emitting only a flashing light, it must emit at least 4 candela... the light shown by the lamp when flashing shall be displayed not less than 60 nor more than 240 equal times per minute and the intervals between each display of light shall be constant.
Given that every light we know of has a steady mode, that means you need a British Standard-approved light to comply with the law. To be fully compliant with the law your bike also needs a front light, front and rear reflectors, and amber pedal reflectors.
In practice, not many bike lights are Kitemarked. The specification for cycle lights dates back to 1986 and is written mostly with filament bulbs in mind; every single light we've been sent for the last few years is an LED. LED lights can meet the requirements but lots of them aren't specifically tested for the ageing British Standard, especially those that are sold worldwide.
Since the Road Vehicles Lighting Regulations (RVLR) was amended to allow cyclists to fit flashing LEDs, we've heard very little about cyclists being stopped for having non-compliant lights. In theory running a non-approved light could be used as an argument for contributory negligence in the case of an accident, though we've not heard of such a case being brought as yet.
What kind of riding will I be doing?
If you're mostly just pootling to the shops and back then something basic will probably do the job. Simple flashers that use button cells or AAA batteries are cheap and effective these days, and they last ages before the battery needs replacing.
If your commute is on busier roads, or you plan to do longer rides at night, you'll probably want something brighter. There's a wealth of USB-rechargeable and brighter AAA-powered rear lights about that will catch a driver's attention from further afield. Many riders who spend a lot of time on the road after dark fit more than one rear light to increase their chances of being seen.
Some rear lights are bright enough to be used in daylight too. There's certain types of riding – racing a time trial on a fast A-road, for example – where you'd want to be running the brightest rear light you can buy. Plenty of city riders run their rear lights in daytime hours too.
Flashing or not?
In terms of the law, it's up to you. The law requires flashing modes to be between 1Hz and 4Hz (one to four flashes per second); as you'll see from the beam comparison engine below, actual modes vary considerably and some fall well outside that. Pulsing constant modes are a grey area.
Ask a rider why they have their light flashing and they'll often argue that it makes them visible from further away. Ask another rider why they have a constant light and you'll often hear that it makes distance easier to judge for following vehicles.
There's not a lot of scientific research to hang your choice on. Most people who run two lights will have one of them flashing. One thing to bear in mind is that if you're riding in a close group – be that a club run, sportive, Audax or anything else – having a bright light flashing in your eyes at close range is pretty annoying. Many lights have low-power steady modes for group riding.
Most rear lights are nice and bright if you're standing directly behind them. But in many situations – and especially for urban riding – traffic may be approaching you from other directions, so it's good for a light to have a wide angle of visibility. Again, the type of riding you do will dictate how important side visibility is to you.
Most rear lights will cope easily with the longest ride you're likely to throw at them, though not all USB-rechargeable ones can be fully trusted to last a whole night, especially on steady beam. If you're planning some big forays into the dark unknown – or if you're just a bit crap at remembering to charge your lights – pick something that has a long run time. AAA-powered lights tend to be the pick for that.
The Moon Cerberus has a three-sided COB (chip on board) design that supplies outstanding side visibility, an innovative hinge system that makes it compatible with all shapes of seatpost, and a dimmer function that allows you to fine-tune your output and battery life.
Side visibility is an afterthought for most light manufacturers, but the Moon Cerberus supplies genuine 270-degree coverage and is as bright from the left or right as it is from the rear. Instead of a curved lens or cutout that is supposed to spread the light from rear-facing LEDs sideways, the Cerberus has three separate LED strips with two of them dedicated to sideways light. For those heart-in-mouth moments when you're passing a junction that's full of dozy drivers ready to SMIDSY you, it's game changing.
The Cateye Rapid Mini is all you could want from a commuter rear light: it's cheap, tough, easy to fit, and provides a good balance between visibility and run time.
For £24.99 (or less if you look around), the Rapid Mini will throw out a claimed fifteen lumens of power across four different modes. You get the standard constant and flashing modes, plus a subtle pulsing mode and a mode that should come with an epilepsy warning. Each of these are bright enough to feel safe (or as least as safe as is possible) on unlit roads at night, making the Rapid Mini ideal for anyone who commutes on more rural lanes. The side visibility is decent too, an important consideration for more urban usage.
Bike maker Giant might not be the first name that springs to mind when you're thinking about bike rear lights, but the Recon TL 200 is excellent. It has useful modes, and puts out plenty of light at decent run times. Tester David Arthur said: "It's become my favourite rear light for all rides, day and night."
On high mode it puts out 100 lumens for 2.5 hours, but the flash mode was favourite, delivering 6 hours using the full 200 lumens. The Recon TL 200 remembers the mode you were using last time, so once you find a favourite, you just turn it on and that's how it's set; no need to waste seconds cycling through the modes.
It withstood filthy weather, jet-washing and being dropped in a bucket of water.
Exposure's TraceR with ReAKT and Peloton rear light impressed us previously without the Peloton technology, but one year on it's gained this extra feature to help keep it near the top of the pile.
In this updated version, the recipe hasn't changed, it's just been added to. You still get a beaming 75-lumen max output, with six modes that dictate how much burn time you have, from 3 hours in the brightest static mode to 24 hours on the lowest flash mode, plus DayBright pulse mode.
ReAKT is probably the best feature of the lot, adapting to the light conditions at the time, and flaring up when it senses that the rider is braking. But now there's a new feature to add to the mix. It's called Peloton, and it recognises when there is a front bike light behind you and dims itself to save dazzling the following rider.
Oxford's Ultratorch Slimline R50 rear light is a simple rear light that does the job of making sure you're well seen, with ample light output, good mode choices, and easy operation and fitting, in a conveniently sized package at a very good price. The R50's COB LED can put out an adequate 50 lumens in its high mode, with medium, low and eco modes available too. You cycle through these with a single click of the power button, and switch them off with a long press. You get 2hrs claimed burn time (mine tapped out at 1:52hrs) on the brightest static setting, and near-enough 6hrs if you have it on the highest flash setting.
If you want a small, well-made, easy-fitting rear light for your bike or bag that's bright and good value, look no further than the Blackburn DayBlazer 65. Its two LEDs pump out an impressive 65 lumens when in the disruptive 'high flash' mode, and 50 and 30 respectively in 'steady' and 'low strobe' settings. Burn times are claimed to be 1:30-6:00hrs depending on the mode, and we found those to be accurate to within around 5-10 minutes depending on how you use it. The single button doubles as an indicator of battery life when the unit has just been switched off, and the light naturally powers down a little when you get close to the end of its life.
The Lezyne Zecto Drive Max 250 has a range of modes that start at sensible light output and increase to the ever so slightly insane with a mighty 250 lumens topping the bill. Each one has its use though and allows you to balance power with battery life whatever the conditions.
Lots of manufacturers are starting to include daytime modes to their lights and this is what Lezyne have done with the Max 250. The 250-lumen flash can be seen a good distance even in bright sunshine and for this reason I'd suggest never using it in the dark as it is downright antisocial for drivers sat behind.
The Moon Alcor is simple, bright and has a nifty magnetic mount. I use a back light almost all the time, perhaps excepting on the finest of high summer days. Bright sunlight, especially when the sun is low and the roads are wet, is not the friend of the cyclist and an attention-grabbing strobe like that provided by the Moon Alcor is a valuable aid to daytime visibility.
That's how I've used this most of the time during the test period, and it was a bonus that the day-flash mode gives over 34 hours of battery life (I never did find out exactly how many hours because I had to go to bed). Moon reckons that on the low-power single-flash setting you should get 70 hours. That's still plenty bright enough for night riding on dark roads, by the way.
The Bontrager Flare R City rear light is a small yet mighty cube-shaped model bristling with sensors and similar tech to deliver optimum light intelligently, whatever the conditions, day, or night.
The tail light has a 100-lumen front sibling, and together they could be all some urban commuters will need. At 26g apiece, they're arguably ideal clutter-free options for summer/time trial builds, or companions for a trainer/audax bike's main lighting.
Brightside's eponymous light is a well-built double-ended side light at a good price that attaches easily to your frame, and gives you an extra dimension of visibility to other road users approaching you from the side. Bright 15-lumen Cree LEDs at each end attract attention.
The Brightside has filled a gap in the market (a quick internet search only unearthed the Brightside and the Cateye Orbit Spoke lightset) in a bid to reduce the instances of SMIDSY (sorry mate I didn't see you) incidents. With too many accidents happening at junctions and roundabouts, the light is designed to give you all-round visibility to motorists approaching from your side – Brightside, not broadside.
A classic that's still going strong, the TL-LD610 has five decently bright LEDs and runs off a pair of AAA batteries. It excels as a round-town attention-grabber because of the mode in which the lit LED scans across the panel: think Knight Rider or Cylon Centurion.
Moon's Gemini is a featherweight USB rechargeable rear light that clips on easily and is bright enough for urban commuting. The small single button is surprisingly easy to find in big gloves.
The Gemini benefits from many of the features of its more expensive big brothers. It has a spot angle of 95 degrees at 10 metres, giving an effective span of 22m. The total angle is helped by the two single LEDs, resulting in a full 360 degrees, very useful for urban riding. The brightest 20-lumen constant is also perfect for partially lit commutes; I prefer a slightly brighter rear for my unlit rides, but this also works well here as a backup or secondary light.
Dr Evil would love it. As well as powerful LEDs, the Sentinel shows riders how much space you'd like them to leave when they pass by drawing a virtual bike lane on the road with frickin' laser beams. Shark not included.
The Knog Blinder Mob Kid Grid Rear Light is a well designed and strong performing rear light. It pumps out enough light to keep you visible and has a really good variety of placement options while also having a good battery life, so it can just be left for weeks until it needs charging.
The light pumps out 44 lumens to keep you well lit without blinding the person riding/driving behind you. In recent years I have come to realise that being stuck behind somebody pumping out 100 lumens from their seatpost is one of my real pet peeves. Sure you can be seen mate, but the only other things I can see are blue dots. I'd say 44 lumens is about the right balance between good visibility and annoying the person behind you.
This super-bright and tough little USB rear light from UK illuminati Exposure pumps out plenty of light and will last for a week's medium-distance commuting (4-5 miles) between charges. It's not cheap, but it is excellent.
Gemini's Iris rear light claims to pump out a retina-melting 180 lumens of red. That's enough to get you seen in any conditions, and there are plenty of lower-power modes for general riding too. And it's well made.
First things first: ye gods, this thing is bright. You know when you turn a light on, and you think, "MY EYES!"? Well I did that with the Iris, only to find out it wasn't even on the brightest setting.
This is the standalone version of the MJ-818 which uses a 3-watt LED for maximum visibility with nine smaller emitters to cover a wide range of angles. In this package it's paired with a 8.4V 4.4Ah battery. If you already have a Magicshine light, then you can get the light and a cable splitter for £31.94
We loved the clever speed-sensing function and incredible brightness of the original See.Sense light. The Icon includes a plethora of extra features linked to a free app so you can control the light on your smartphone. This nifty little blazer will also tell your loved ones if you have a crash, and alert you if someone tries to make off with your bike when it's parked up.
The Icon uses super-bright Cree LEDs putting out a total of 300, which certainly count as bright enough for the old joke 'do not look at laser with remaining eye'. This is not a light to turn on while looking at it – it is ferociously bright.
Another light with value-added smart functions, the Rotlicht acts as a brake light, brightening when you decelerate, and has a light sensor so it can adjust its output to the conditions. Clever stuff.
The latest version, the Rotlicht Max, has twice the run-time of the original Rotlicht.
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Dave is a founding father of road.cc, having previously worked on Cycling Plus and What Mountain Bike magazines back in the day. He also writes about e-bikes for our sister publication ebiketips. He's won three mountain bike bog snorkelling World Championships, and races at the back of the third cats.