No one says "the I-5" — it's just "the 5".

'The 26' is not something people from Oregon say.

I’ve lived here for 23+ years and have heard people say it all of that time.

Lots of transplants from points south say it, or people who have spent time with them.

So, basically everyone in the metro area? Because in the past two decades, I’ve never heard someone say 26, it is always the 26.