From The "Wish I'd Thought Of That," Department - Sigma Automatic Brakelight For $10 USD.

All photos courtesy of Sigma

Now, here is an idea so simple, it is a wonder no one thought of it, sooner.  An ingenious, mechanically operated, light which automatically illuminates when the rear brake is applied.  It mounts in seconds, runs off a tiny, replaceable CR 1025 button-cell battery, and is cheap at $10 USD.  What's not to like?

The unit itself mounts to the rear brake cable, via a hex clamp, up against the adjustment bolt, and when the brake is applied, it squeezes the unit, triggering the light.  Release the brake, and the light switches off.

While it is being marketed as an aid to motorists, I see the larger service being to other cyclists, and hopefully, it will help to eliminate the rear-ender in pacelines. 

Safety never looked so simple and cost-effective.

Clicky here for the Sigma web site.

Here is the installation video, courtesy of Sigma.