I have a gripe with this. Specifically, the southern extension from Lafayette to New Orleans. I don't disagree with the need for the upgrade of US 90 between those two points to Interstate standards - those parts of Southern Louisiana are languishing, in part due to being so far away from an Interstate. No... I disagree with the number for it.
Extending I-49 south and east along that route would create a numbering violation, since I-49's "southern" terminus would be east of I-55. Also, it would be confusing, direction-wise. I-49 northbound would start by traveling SOUTH! If they tried a Michigan solution (something like I-69 going from signed N-S to signed W-E), then the exit numbering would go backwards, increasing as you go west.
I don't take credit for the number it should be, but I do take credit for the sine.
There's some good reasons why I-49's southern extension should be I-6. First off, it would fit nicely in the grid. It would exist wholly south of I-10. Latitude-wise, it would exist between I-8 and I-4. The small jog back westbound (assuming US 90 Business (Westbank/NO)/I-910 was used) would be nowhere near as confusing as the long southwesterly jog down to Houma and Morgan City before taking a northerly slant to Lafayette. The exit numbering could then run in a conventional direction, from west terminus to east terminus. As well, there is precedent for two Interstates in Louisiana sharing a terminus. I-12 and I-59 share a common terminus in Slidell. So too could I-49 and I-6. The fact that the two Interstates would be on the mainline going north-south through the cloverleaf that is I-49's current southern terminus means one wouldn't have to weave to "stay on the Interstate."
So in conclusion, down with I-49 South. Long live Interstate 6!