Federal Highway Administrator Victor Mendez was in the Valley today to announce the designation of 77/83 from Harlingen to Palview as Interstate 2 as well as officially announcing the designations of 281 from Pharr to north of Edinburg as Interstate 69-C and 77 from Brownsvill through Willacy County as Interstate 69. The Rio Grande Valley no longer has the dubious distinction as the most populous region of the country without an interstate highway.
Now, whether it is legal or not is a question for TxDOT, DPS and whatever LEO catches up to you!