I agree with Wayne about the chance of not hearing a vehicle coming from behind. It really does require unusual wind or a very quiet vehicle.  I did have that experience once with a Prius when it passed me, slowly, using its electric motor. That scared me because I was about to make a left turn and hadn't noticed it using my hat mounted mirror! There certainly will be more opportunity for that situation to occur as  more  electric vehicles are used on or roads.

Now, my policy is to always signal, with my arms, before I turn, even if  I don't think there is a vehicle close behind me. I also always wear brightly colored clothing and have reflective material on my bike and/or on my body when I'm riding. I also have rear flashers on my bikes, but only use them when I think they are  needed because of low visibility riding conditions.

Good to hear that this is in addition to a mirror.  I think we all need as many safety devices as possible now days especially with lots of nearly silent vehicles coming on the scene.  I personally could not cycle without my mirror.  As I said, it takes literally a split second for me to see what's going on behind my back, way faster than turning around to see.  Also, turning around to see what's behind you means that you take your eyes off what is in front of you like that cobra speeding across the road which you are suddenly on a collision course with.   Split seconds count especially when you are moving at 10 or more miles per hour.

How does this work when another bicycle approaches from behind? Will Scott need to ride even farther behind Rocky? (First question is real. Second question is joke.)

Jacinto says no bicycles riders have caught up with him (joking), but he says it  beeped when a dog was chasing him. Let's see if any non wise guys answer.

Most of the time the unit filters out anything smaller than a motorcycle - it will occasionally miss a motorcycle but very rarely alert on a single cyclist. A group of 3 or more close together can trigger it. Not real useful in large group rides, obviously!

I've only had one dog chase since using it, have no idea if it was beeping while I pedaled like crazy, but I doubt it - all I remember hearing were dog claws clicking on asphalt!

Very funny.  All will be revealed when I put up a product review in the next few days.

Thanks for the review. I won't ride without my little clip-on mirror, but this sounds like it might be another helpful layer of safety. Meanwhile, I'm still looking for a reliable bike-portable anti-automobile force field. Shields up, Mr Worf!

De rien, Monsieur Grumbleface!  Let me know f you find that force field and order one for me too.  I wouldn’t get rid of your mirror either, nor will we.  It partners well with this device.

And for anyone else still following along, here’s our last word on this thread: the promised product review.

A fun hack for Varia Radar owners that use Wahoo Elemnt/Roam/Bolt GPS bike computers:

If you are really bored or are actually tired of the little car icon that shows up on the side of the display when cars are behind you, you can change that to anything you want!

How to do is explained in detail in this video.

It is easy to do, the hardest part is creating the tiny (25 x 25 pixels or so) images, but that author of that video has several you can use. I figured out how to do it, made one of our old dog, so she chases me now.

Can you upgrade it to use context-sensitive icons so you know what’s behind you - cars, bicycles, dogs, cougars?