Automatic brake lights that only come on when your bike is slowing and stopping aren't a new idea. But the folks at Sigma have come up with cheap and clever solution so that their $US10 LED bike brake light doesn't need a pricey built-in accelerometer.
Installation is as easy as clipping the light onto the mechanical brake cable just above the bike's rear caliper, and tightening it in place with an allen key. Every time the brakes are applied and the caliper squeezes the bike's back tire, the LED light is squeezed as well -- and glows red to let other cyclists and drivers behind you know you're slowing or coming to a stop.
The water-resistant light weighs just seven grams, and it's powered by a replaceable button cell battery that's good for about 100,000 uses. If you live out in the suburbs it should last for years on a single battery. But if you live in a big city, expect to replace it every few months.