I'm not sure that such a "NAFTA Superhighway" is even being planned. One FReeper, however, has said he's heard economists talking about that; they could be misled, for all I know. Also, the spp.gov website does mention that at least one working group is studying mult-modal corridors.
The only similar projects that I know of that are actually in the planning stages are TTC-35 and TTC-69 (Trans-Texas Corridor), and they're strictly in Texas (for now).