A mirror is better in many ways, IMO, like being able to see the lateral position of approaching traffic and what the vehicle is, whether it has a wide trailer, etc... And my blinkie is on at all times, and bright enough to be obvious at 1/2 mile in broad daylight. Now, if the radar indication is prominent enough to alert me of approaching traffic without having to look down to check the indicator it could be useful adjunct to a mirror.