Would you consider this useful?

[Gunslinger]

Nope, not useful at all. Wouldn't buy it. Anyone who relies on something like that to keep them safe is going to get messed up bad, or worse. I respect you guys for working on new ideas though.

And I'm in total agreement with BuzZz on the new cage technology. This buzzer will go off if someone's in my blind spot so I don't need to check anymore. Meh.

[Flesher]

I think it would be extremely useful... for cars that is, and instead of detecting vehicles in a car's blind spots instead activate the system to detect oncoming traffic ( and sensitive enough to detect motorcycles ) when a turn indicator is engaged.

[fireguzzi]

I still say if it is unobtrusive it would make a good aid. Not something to be relied on. Just as a heads up to do a head check in case you may have been otherwise distracted.
Another idea would be left and right tones played through speakers in the helmet.