Dr. Gooch looked it up and thinks they may be gnat larvae. They feed on organic matter. In this case, the mulch around our oak tree. Yesterday morning around 8am (and again today) they made their way from the grass across the driveway. Strange route? Maybe. Gross? Definitely. What a bizarre phenomenon. They traveled en masse using each other to inch across the asphalt. Most were run over by our traffic (which I am perfectly fine with). Ew.