The gadgets would have to meet an impossibly low false-positive rate and they'd need to be mandatory and non-removable for this to be effective. Constantly impaired drivers won't voluntarily install the devices, and any non-impaired driver who is falsely prevented from using their car will get rid of the device and not care about the insurance discount.

Also, they really need to be able to detect the other really bad cause of impairment: mobile phone usage.