Why Are There So Few Actually 'I' Interstate Expressways in Metro Houston?

[bwana39]

Would be interesting to know if there were any other plans for extending I-45.
Also, why doesn't Texas like 3 digit spur/loop interstates?

It is generally believed that when the updates to US-75 in downtown Sherman are complete, that Texas will ask for it to be extended to at least the US-69 / US-75 intersection near the Oklahoma state line. Oklahoma is sort of thinking about it, but honestly they already have problems maintaining that which they already have.

[bwana39]

2)  Why is I-45 'I-45' and not some non-two digit I?

It wouldn't fit in the grid elsewhere because it would duplicate US-45 in some states.  So they "wasted" it in a single state, rather than skipping it altogether as with I-50 and I-60.

If I-45 was on the 29/49 corridor, there would be no duplication with US 45.

I-49 was first proposed DECADES after I-45 was built. Besides, I-49 originally went only  from Lafayette LA to Shreveport LA. Proposals to extend it came about a decade or more after the original stretch was opened.