I could only see this being useful on the type of suburban roads or highways in the US or Australia that I see people ride, and I used to ride when living in the US.

Here in Europe I only have 3 riding scenarios, neither of which those radars would be useful:
- either I am on the road in the city, and the thing would go bonkers because there is 10 cars behind me at all times.
- or I am on a separated bike lane, and the cars behind me are a non-issue.
- or I am on a 'backroad / farm road / gravel track' where there is *almost* no traffic anyway.

There are occasional instances where I am on a big road shared with cars, but those are quite rare.

So I see their usefulness, but it really depends on the types of roads in your area I think.