> If someone told me "I took 5" I'd expect them to be talking about some kind of quantity and wait for them to finish the sentence.
I think you're thrown off because 5 is such a low number. Most highways have higher numbers, so it looks more natural without the "the".
Using Dallas highways as an example, "I took 635 to Preston". Or maybe chain some highways "I took 121 to 75, then to 635, and I got off at Royal".
The only highway in Dallas we use "the" for is the Dallas North Tollway, which most people just call "the Tollway" (NTTA is currently trying to push an acronym, "DNT", but nobody uses it). And I've seen a few people refer to President George Bush Turnpike as "the Bush", which is uncommon enough that I do a double take whenever I hear it (it's usually just "Bush" or "George Bush"... and maybe "190" from people who remember when it was under construction before they decided to turn it into a toll road).