State Transportation Flexibility Act This bill provides flexibility for a state to opt out of the federal-aid highway program. Specifically, the bill requires the Department of Transportation (DOT) to implement a direct federal-aid highway program that permits a state to (1) waive its right to receive funds under the program, and (2) receive a prorated amount of taxes apportioned to the Highway Trust Fund (other than for the Mass Transit Account); sets forth responsibilities for states that elect to waive their rights, including that such states must agree to obligate or expend program amounts exclusively for projects that are eligible for surface transportation program funding; and directs DOT to implement a similar alternative funding program for public transportation programs.