Washington Monthly: Six days before Thanksgiving, a truck driver heading south on Interstate 81 through Shenandoah County, Virginia, ploughed his tractor trailer into a knot of cars that had slowed on the rain-slicked highway. The collision killed an eighty-year-old woman and her one- and four-year-old grandchildren, and brought traffic to a standstill along a ten-mile stretch of road for the better part of the afternoon. It was a tragedy, but not an unusual one. Semis account for roughly one out of …