This is not news. Seriously. Cowboys have used this trick for decades when they're in their pickups driving around all over creation. (They don't drive over the clothes to wring 'em out, though. I made that up.)

http://www.associatedcontent.com/article/473033/ecoliving_truck_bed_clothes_washer.html
for example. Why go to the trouble of hand washing if you're driving over rough terrain?