<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by Canarsie:
For example, all one-digit and two-digit Interstate highways would have to cross at least one state boundary to be a true Interstate highway.</font>
This is not an inconsistency. It is an example of the errors created by not paying attention to capitalization.
An interstate highway (lower-case "i") is a highway that crosses one or more state boundaries.
An Interstate highway (upper-case "I") is a highway that conforms to the requirements of Public Law ..., yadda yadda. Those say nothing about crossing state boundaries.
Requiring an Interstate highway to also be an interstate highway makes as much sense, with all due respect, as requiring everyone named Potter to work in ceramics. There is a historical connection in the origin of the name, but that's all.