For the last decade I've been an avid bike rider. When I was still working in an office a co-worker started riding his bike in, and I figured out there was a 9 mile route I could take that larger avoided busy roads. The last summer I worked in an office 5 days a week (2013), I road in 27 times. It was great.
Switching to largely work from home, with a rail trail 3 miles from the house, means I can get out and ride regularly. I have a standard 12, 18, and 24 mile ride based on the time I've got in any given day (I'm pretty much 12mph on my bike, so that's convenient blocks of time).
The only real downside of riding from the house is we live on a hill. So in the last 1 mile of riding I've got about 200 ft of climbing. Which is also when I am most tired. The first part is on a road without much shoulder. About 2 months ago when riding home, I got wizzed by a car I didn't realize was behind me that had drifted a solid 6 inches over the white line. That had me freaked out a bit. An unrelated set of google searches doing wish shopping for bicycles made me discover bike radar units are a thing.
Enter Garmin
Garmin, makers of GPS systems, have a bike radar unit that's integrated with a tail light. The Varia 515 (that I got) sees back roughly 500' on objects that are closing on you. You can pair it with a head unit, or a phone. I splurged and got a Garmin bike computer as my road bike has been lacking a functional bike computer for 15 years, and my birthday was coming up.
First Impressions
This is really good. I've done 3 rides now since I got it, and it sees every car, often well before I can recognize them in my mirror on my helmet. Today I had an instance where I thought it false positived on me, but then I looked harder and nope, there was a car coming up on me.
The interface on the Garmin 840 that I've got is as soon as a car comes into range, the screen flashes red, beeps at you, and then shows you an overlay of a series of dots coming up the right side of the screen. That gives you a relative sense of distance.
I've seen it track up to 3 cars at once (that may be the max). It has picked up ever single car behind me, well before I knew they where there for other reasons.
It's really good. It's better than I imagined it would be. And it's definitely given me a lot more confidence to be out on the roads.
Definitely worth checking out if you are an avid rider. It sucks that to be safe as a cyclist with distracted drivers in 3 ton mall crawlers you need gear like this, but it's where we're at. So highly recommend.