Working somewhat similar to your basic automotive reversing sensor, Garmin's new Varia Rearview Radar is designed for cyclists to easily be more aware of their surroundings.
Coming complete as two units, the Varia Rearview Radar gives riders a chance to know what's coming up behind them, be it man, machine or both in one. Sensing cars up to 153 yards behind the bike, this unit utilizes LED lighting to display the distance between car and rider, alongside changing color when the hunk of metal gets closer.
With the colors transgressing from green to amber and finally an ominous red, this unit can also tell the difference between traffic travelling head-on with the rider and parked cars alike. Priced at around $200 USD, this can be bought direct through Garmin themselves for the tail light only, or bundled for $299.99.