It's called "lane splitting" or "filtering", depending on where it is. It's legal in California but nowhere else (I think) in the US.
I'm a fair weather rider and would do it at stop lights, maybe, but not in moving traffic.
Things can happen...even on an idyllic country road. The ONLY time I ever rode like that was Route 15 in Gettysburg...I was going to be late for the 135th Anniversary Re-enactment!