Why are trailers 53 feet?

Answer 1

The Surface Transportation Assistance Act authorized trailers 53 feet long to be operated on the national network of highways, and encouraged states to allow them on all roads. The intent was to improve the cargo carrying capacity of a truck: a 53-foot trailer will carry more freight than a 40-foot trailer, which was the pre-STAA maximum.

Here's where it gets fun: states can set a measurement called KPRA, which means "kingpin to rear axle." For instance, California requires a trailer with two or more axles to be 40 feet or less KPRA on a 53-foot trailer. The kingpin--the metal piece that locks into the fifth wheel on the tractor--is four feet back from the nose of the trailer and you can't move it. Therefore, the center of the rear axle has to be at least nine feet from the DOT bumper in the back of the trailer. If the freight you carry is both heavy and bulky, this is going to prove extremely entertaining because if you load past, say, 36,000 pounds you are probably going to be overweight on your trailer tandems. Guys who pull only in California should consider running 48-foot trailers rather than 53s, because there's no KPRA measurement on a 48. My brother pulls into California and they run 48-foot reefers for exactly this reason.