Garmin Rearview Bike Radar
When you're on your bike, your eyes are normally — and rightfully — on the road in front of you. The Garmin Rearview Bike Radar gives you another pair on the road behind you. It uses a rear-facing radar unit to detect oncoming vehicles up to 450 feet behind you, and flashes warnings to both you and the vehicle with increasing frequentness as they get closer. It's not quite as good as a second set of eyes in the back of your head, but it's as close as you can come without really invasive, very likely illegal/fatal surgery.