Why are truck trailers 53 feet long?

The Surface Transportation Assistance Act of 1982 allows trucks pulling trailers 53 feet long to travel on roads designated as part of the National Highway Network, or any road within one mile of the National Network, or en route to a terminal which includes a shipper, a consignee, a firm engaged in the transfer of goods, or a trucking company's facility. (The easiest way to find out if a road is on this network is to get a trucker's road atlas, where the National Network roads are marked.) I think they came up with this number because a conventional tractor with sleeper cab connected to a 53-foot trailer produces a vehicle 70 feet long.