Garmin's Varia RVR315 Rearview Radar detects traffic from up to 140 meters behind your bicycle. This seatpost-mounted indicator connects to Garmin's Edge Computer, and you can connect it to your smartphone via the Varia app to increase your awareness with vibration alerts, sound tones, and even graphics. The device is also third-party compatible so you can connect it with GPS apps to map the radar's alerts. Garmin packed the compact radar with a 7hr battery, and it's small enough to mount to most seatposts.