My son's been suggesting that a Garmin Varia might be a good X-mas gift for him. I've known about them ever since they came out and have read DC Rainmakers article on them.

Do you generally get at least 5 seconds warning on traffic when you are on roads with 50mph plus traffic?

How useful in slower city traffic is it?

My son thinks it'll also tell him about other cyclist coming up behind him? I can't find anything conclusive on this. I know Garmin only talks about cars and such. I'm thinking maybe it will if there is a large overtaking speed.

Any of you have experience with it coupled to the Vivosmart HR?