Why not just use a side mirror? I use one and it is so convenient

The Varia can give you auditory alerts so you don't have to take your eyes off the road and look in your mirror. In addition, it can tell you how many cars are behind you with a range that would be hard to see in your mirror.

Varia can supplement bike mirrors with an early warning that a vehicle is approaching. Some people don't like bike mirrors (they look dorky), but personally I feel half-blind when cycling without a mirror.