Not legal, I'll bet.
We had a woman lose her face a decade ago from a 2x4 coming off someones truck.
You've heard of DUI Patrols, and such. Here we have signs on the highway announcing Unsecured Load patrols.
"Every state has laws that make it illegal for items to fall from a vehicle while on the road. In Washington State, the fine for transporting an unsecured load is $228. If an item falls off a vehicle and causes bodily harm, the driver faces gross misdemeanor charges and penalties of up to $5,000 and/or up to a year in prison. King County's unsecured load fee charged at solid waste facilities is in addition to any penalties or fines by law enforcement officers."