Originally Posted by Seattle Forrest

I think the head unit just has to be powered on. It tells me about cars approaching before I start a ride, eg when I'm calibrating my power meter. Most people aren't going to spend hundreds on a head unit and not use it, so it's almost functionally identical to what you're asking for in the sense that you don't have to do anything additional that you wouldn't do anyway.

I'm not telling you to buy the Varia, it sounds like the Reflex Auto may be better for your needs. The only reason I'm posting is to clarify how the Varia works. So that you or anyone else who finds this thread has good info to make your or their choice on.