If a bridge is rated at say, 60,000 pounds, I wonder if that means at any conditions. If you're old enough to remember throw rugs on waxed wood floors (before the days of polyurethane finishes), you probably remember what happens when you stop walking...you and the carpet keep going. I've noticed rural roads that were wrinkled where big trucks approached a stop sign, they pulled some blacktop along with them. If you get out and examine blacktop roads where there's a sharp bend in the road, you can usually see cracks where the road is being slung outboard by the force of traction of trucks making the bend. So if a bridge is rated for a truck crossing it, you can understand that a truck slamming on his brakes on that bridge puts 3 times as much strain on it.