Everyone is aware of the requirement for a CDL for vehicle combos over 26,000# GVW. Not too many are aware of the requirement that any vehicle combo of 10,001# and greater is required to have a DOT number. Enforcement will be phased in by states over the next several years. Some states have even lower weight requirements for a DOT number designation. And the regulations are based on listed GVW's of truck and trailer, not the actual weight. The most common lawn trailers have two 3500# axles, making them 7000# GVW vehicles. Is the GVW of your truck that pulls your trailer less than 3000#? Hardly, LOL. Paper work seems hairy just to get the DOT #, and you may have to search for someone in your state offices to get you on the right track. But some of the state commercial vehicle inspectors are well aware of this requirement.