Since you already have your answer, I will just post this for others who may want to click a link rather than spend hours on the phone with government bureaucrats who often don't know the answer themselves. :)
University of Vermont has a web page "Riding on Public Roads." It lists rules/laws from several different states. Some may find this useful. Riding on Public Roads
Keep in mind, however, that being "right" will not help you if that SUV or delivery truck hasn't also read the rules stating you have right of way. Your horse may dent the truck, but the truck will still win the encounter. Always apply a good helping of common sense to the rules.