US Route 101 — "The 101"

It's a regional variation. In California, where I grew up, it was "take the 101 north", "get on the 405", and so on. In Pennsylvania, where I now live, it's "take 202 north", "get on I-95", and so forth. Note that this only applies to numbered roads, really -- in both places, named roads get the article: "the Schuylkill", "the Hollywood freeway", "the Blue Route", "the Pacific Coast highway".

I don't know where the dividing line is for the different usages, nor do I know what they do in other English-speaking countries.

My husband reports that local usage in Indiana (where he lived for a time) does not use the definite article on numbered roads. Friends who lived in Florida report the same thing.

According to Wikipedia, this name for U.S. Route 101 is localized in southern California. Apparently this way of naming highways is a common phenomenon in Los Angeles. More common in the States are affectionate nicknames for highways without using their route numbers. Some examples from the Boston, Massachusetts area are, The Pike and The Artery (pronounced Ah-taree).

It's called "the 101" because that's short for "the 101 Freeway", which is its real name.

Here's how the L.A. Times refers to it:

The driver of a big rig that slammed into the center divider of the 101 Freeway in Studio City during rush hour Tuesday morning was arrested on suspicion of drunk driving, the California Highway Patrol said. [Emphasis added]

By the way, the 101 is also called the Pacific Highway in some parts of California.