Why do many tractor-trailers have a 53’ posted at the front end of either long side the trailer? What’s the significance, and is it important enough to post?
That is the length of the trailer - typically trailers are 53 feet or 48 feet.
It’s most likely the length of the trailer. 53 footers are the most common box and reefer trailers currently in use. There is a push by some in the trucking industry to allow 57 footers, but so far it hasn’t gotten enough support. 53’ trailers are more restricted in the routes they may use than the older, but still common, 48 footers.