It's been my experience that pretty much all wildlife gets by in life by watching out for bigger predators and getting out of the area. In this case, you would constitute that larger predator. While I guess it's possible to accidently step on a snapping turtle while wading, I think it's highly unlikely. My guess is that the turtle you found is a female out looking for a place to lay her eggs. I live on a small (50 acre) lake and there is one VERY large snapping turtle that I see from time to time while I'm out in my kayak. His head is about 2/3rds the size of a football and his shell is probably 24 inches across. He has surfaced right beside me a couple of times and quite frankly scared the %$#^ out of both of us. He dives immediately and gets out of Dodge. All that being said, large snapping turtles are the apex predators in small lakes and ponds and I would not want to handle one even as large as the one you found on your driveway.

Jim Smith