I think, it is a way to say rather than idiomatic phrase, because any verb can be used instead of "поехали" with the same sense "let's do it right now".
Yes, something like that. It is like an imperative cooperative ("let us do it") with the idea of immediateness ("right now").
- Едемте в лес! (imperative cooperative)
- Едемте.
- Ну так поехали!
- Let's go to the forest!
- Let's go.
- So, let's go right now!