Sure glad it all ended okay. I used to live near a very busy major road and I always made sure to never walk Lucky near that road in case he might be too familiar with it and if he ever got loose try to go that direction. Not sure if that is the right strategy or not. Must be more challenging though if it is at the very end of your driveway, because I guess you are sort of limited then to walks on your property or walks in parks that you have to drive to.
That said, my mom's Lhasa used to dart out whenever she could, but she did not really get regular walks. Lucky and Sparky get at least two twenty minute walks a day and that seems to easy their need to dark and explore. I dropped Lucky's leash a couple times and I've been able to just remain calm and pick it up. It amazes me (and I don't take it for granted, becuase I know the results might be different next time).
I also usually try to crate them or confine them so they cannot get to the door whenever I have to open it. Lately however, I haven't been 100% about this because they've been so good. I think I'll take your experience as a cautionary tale though and make sure I do that in the future. It is not worth taking any chances is it? I am pretty sure if my dogs saw a squirrel they'd be after that squirrel regardless how busy the road was.
(a.k.a. Lucky's & Sparky's mom!)