When the US highway numbers were assigned in 1927, US 81 was the only route initially serving Waco. But US 77 soon joined with it, when it was extended down from Dallas in 1933. These routes were concurrent south of Hillsboro, and through Waco they followed New Dallas Highway, Hillsboro Drive, Elm Street, Washington Street, 18th Street, and Circle Road, to the Circle. From there they diverged, with US 77 following Robinson Road, and US 81 following the corridor now occupied by I-35.
(Most recently updated 10/15/2019.) At 568 miles*, Texas 16 is the longest state highway in Texas (that is, the longest road with a state highway designation, as opposed to a US route or an interstate). The second-longest is Texas 6, at 551 miles*. These two routes intersect in the town of De Leon. Actually, they overlap for about a half-mile: