Ruby's outings, Am I doing this right?

Today was the first of many (I hope) outings to try to  get a handle on Ruby's reactivity.  We made a short trip to a local rail trail.  Basically we sat in the open back of the car and I fed her cheese every time something moved (car, person or dog).  When we started she was panting heavily, whining  and wagging a high, stimmed wag.   By the time we left her mouth was mostly closed and her tail was down some (still wagging though).  We never got out of the car (although she wanted too) and I let her have a limited range of movement (she could sit and down but other wise not move around much).  Part of her excitement was anxiety but part of it was the fact that I was feeding her cheese.
 
I guess my question is what exactly am I looking for here?  How do I know we were successful?  Should I have let her out of the car?  If we are looking for calm(er) behavior should I downgrade the treat to something a little less exciting?
 
I feel like I'm reading a new knitting pattern.  I think I understand all the words but it doesn't quite mean anything yet.
 
All guidance is welcome!