Why are brake lights not common? I spent a fair bit of time riding in the lane, in traffic. In this situation, it seems that a brake light would be very useful: It's not always obvious when the vehicle ahead of you is slowing down, and I don't want to be rear-ended by someone who doesn't notice me slowing.

This intuition does not match reality. I was not able to find any bicycle brake lights that seem really ready for prime time. This is what I was able to find:


Q-Lite Multi — Apparently available from just one vendor (TerraTrike). Not mentioned on manufacturer (Q-Lite) website.
Maxxon — Available on Amazon, but poor reviews suggest it does not detect deceleration well.
Revolights — Complicated spoke mounting system; unclear drivers will interpret it properly.
Velodroom — Not yet available (pre-order only).
LucidBrake — Perhaps the most promising of the bunch. Apparently available only direct from a not-so-great website.


The first uses a brake handle-mounted switch. The others use accelerometers to detect slowing.

None of the major bike light manufacturers seem to offer a brake light.

My question is, what's the disconnect? Am I misjudging the value of brake lights? Is there some challenge that makes them hard to offer for bicycles?
Summary: it's hard to make a light that reliably turns on only when the brakes are used.  Most cyclists who want brake lights buy rear flashing lights because they're cheap and ubiquitous.

Ignoring cost, to work well a brake light on the back of a bike would need to be paired with a constantly on, non-flashing light, purely so that people who saw it would recognise it as a brake light. Otherwise it's just another blinking light on a bike. Then it needs to be daylight visible, or no-one will see it.

From my experience with rear lights, 200 lumens is about the level you'd want from a brake light. My always-on dynamo rear light is about half that and while it's visible it doesn't really stand out. In other words, it makes the bike more visible without being too bright. But for a "hey, I'm stopping" light, you want more than that. So the combination of a 200-500mW always-on light with a 1W or 2W "BRAKES!!!" light would probably work pretty well.

Unfortunately most super-bright LED rear lights are designed to overdrive the LEDs in short flashs, but for a brake light you want a solid 2-5 seconds of on time. Which means bigger, more expensive LEDs. And also a bigger, heavier battery to drive them.

The sensors are also challenging. SeeSense have done a lot of work on accellerometers in bike lights and from using their light, braking detection is ok but not brilliant. If I  hit the brakes to reduce the impact of hitting a pothole, for example, the light seems to got into "holy shit" mode instead of braking mode 90% of the time. Which might be ok for a brake light, 5 seconds of braking mode in that situation is fine. But it means that I'll get 5 seconds of braking mode when I bunnyhop a curb or bounce a speed bump too. Which isn't really "braking" as I think of it - I'm definitely not slowing down at that stage. So you probably want sensors on the brake levers as well (the accellerometer is built into the CPU that runs the lights). Which means wires and fiddly mechanical bits. I dunno, that seems hard to me, you have to cover road bikes with integrated electronic shifting right down to basic V brake levers. Sounds expensive.

To get an idea of costs, look at some of the KickStarter bike lights. It seems to cost about $100 to get a light to market. Which means hardware costs of about $25. If we double that because we're using, say, two 18650 LiIon batteries (18Wh) instead of one, CR123/16340 battery (half-16550, 4.5Wh) and a 0.5W RED LED as the primary light plus a pair of 1W LEDs as the brake light... ok, maybe triple it. So you're looking at a light that will cost about $200-$300, assuming the sensors are cheap and reliable. Cheap means pure accellerometers, like the last light in your list. 

Which makes them "something is happening" lights... just like any other blinking light on the back of a bike.