The absurdity of this is that drivers licensing is not federal. I don't see many people driving their tractors across state lines. In fact, it has been difficult for the federal government to impose any requirements for traffic laws on the states, except under threat of loosing highway dollars. That is the only way they have been able to get the states to come into line on speed limit regulations. Constitutionally, they only have the power to regulate interstate commerce. That is how they regulate auto manufacturers. Cars could be transported and sold across state lines, and pollution is carried by the wind across state lines. If they were to impose this, I can guarantee a challenge by many farming groups. And if the Department of Agriculture actually represented farmers, instead of corporate groups and special interests that have noting to do with Ag, they would never let anything like this fly.