Just a minor note from a source usually reliable on well documented uncontroversial matters. On the other hand, it also notes I-40 through the TX Panhandle is among the few at-grade interstates regulations also forbid, a far more significant infraction even in sparsely populated areas. My memory of "at grades" meaning was foggy, but further reading reminded me: There are no feeder roads or even entrance/exit ramps; in places, Wikipedia claims residential driveways abruptly meet the interstate at their terminus. Nothing like merging from ones driveway to a 70 mph interstate.