Earlier this month, we first heard about a radar system for cyclists, known as the Backtracker. In a nutshell, it emits radar pulses to the rear of the bike, then warns the rider when it detects vehicles approaching from behind. Although it's not yet commercially available, I recently had the chance to try out a pre-production review unit. It definitely shows promise, but could perhaps use one tweak.
Read the full article: Review: Backtracker radar system for bikes