There's a phenomenon commonly called "truck suck" that occurs after a semi-truck or other large vehicle passes you at speed... They're pushing a lot of air out of their way, and the initial wind blast if they "buzz" you pushes you to the right (toward the shoulder); as soon as the cab has gone by air rushes back to fill the partial vacuum, and can pull you right under the trailer and into the rear wheels. Even a 3-foot passing distance doesn't mitigate this all that much. It takes firm control of the handlebars and rider balance to counteract this. We have more than our fair share of pulpwood and log trucks out here in south Arkansas, with an 81-year-old rider on the shoulder on the fog line, I'm pretty sure that "truck suck" is what got him.