The "The" Debate September 21, 2013
Having grown up in Southern California and living in the Bay Area for almost a decade, I witnessed first hand the "the" debate. Do you say "the" before stating the freeway? Is it "The 101" or "101"?
What's interesting is that I never knew of this debate until I moved to NorCal. Nobody in SoCal even cares. In NorCal, they'll automatically know you're a transplant when you say "The 101" or "The BART" and consequently ask you about it. This is really a one-sided debate.
But to understand why SoCal prepends "the" to the freeway, you need to understand the actual freeways in SoCal. SoCal is blessed to have two cross-country interstates - I-5 and I-10. NorCal, which is the Bay Area for this debate, has none.
SoCal freeways require additional specificity
You can't say, "take 5". That is not specific enough. Take 5 what? Minutes? Dollars? But when you add the "the", it's automatically understood that you are referring to freeways. 405 is a time. The 405 is a freeway. After all, southerners are talking about freeways 50% of the time.
On the contrary, NorCal has US-101, I-280, I-80, etc. These numbers are pretty specific, whereas the numbers 5 and 10 are not. The additional specificity is not required, so you can say, "take 101" or "take 80". You rarely hear anyone say "take 101 minutes" or "take 80 seconds".
Other forms of additional specificity
When you only refer to freeways by their numbers, the "the" makes it understood that these numbers are referring to freeways. There are two other ways to add specificity.
The first is the type of freeway. "U.S. 101". "Interstate 5". "Route 66". With this added specificity, "the" is no longer necessary. "Take the Interstate 5" doesn't sound right. "Take Interstate 5" does.
The other form of specificity is direction. "Take 5 South" sounds right. But what about "Take the 5 South"? Personally, I don't think it sounds right, but I probably say it. I think it depends on how people use this.
Some consider "5 South" to be a compound noun. In other words, the "5 South" is the entire Western portion of I-5 spanning from Washington to California. Others consider the "South" to be an adverb, so they are saying, "Take it south". In this sense, they are referring to the 5 as the entire freeway, both western and eastern portions, and south is the direction you go on it.
When you say, "Take 5 South", you can be using South as either part of the compound noun or as an adverb. When you say, "Take the 5 South", you are referring to 5 South explicitly as the specific part of the freeway. And so comes the next part.
Directions vs. Routes
SoCal's freeway system is a very complex. It's a grid, and most southerners know a decent amount by heart - because there's no public transportation. You need to know the xx0 freeways go east and west and that the xx5 freeways go north and south. You need to know where each freeways intersect. NorCal's freeway system on the other hand is simple. It's basically a loop with US-101 and I-280 on the west and I-880 on the east.
So southerns think of freeways as lines that intersect. When they are told and figure out the lines to take, they can easily figure out the directions. Northerns think of freeways as directions because they already know which lines to take.
Directions and Delimiters
Due to SoCal's complex freeway system, you generally take multiple freeways to reach your destination. In NorCal, you generally only have to take a single freeway.
For example, the directions from Pasadena to West LA would be 210W to 110S to 10W. Now how would you say with just numbers? "210 to 110 to 10". That sentence was literally all numbers! You said "210" twice. Imagine if you were giving directions on the phone. Thus, we add "the" before the numbers to make the directions easier to hear. "The 210 to the 110 to the 10". The other person would be very confused otherwise. Of course, you could talk slow, but southerners aren't slow like northerners.
How about specifying directions? "210W to 110S to 10W". That didn't help that much as I still hear two "210"s. This is another reason southerners don't specify freeway directions.
What about specifying the freeway types? "I-210 to Route 110 to I-10". Sounds right, but "the" is so much easier to say and think about. Nobody wants to think about whether a road is an interstate, a route, a parkway, a highway, etc.
So you probably understand my conjecture: areas that don't have common numbers as freeways don't use "the", and areas that don't have complex freeway systems don't use "the".
Ask Washington and Oregon what they think - they probably don't care because they have Interstate 5. But the Bay Area doesn't, and that's why they probably care - they feel left out.
I don't know anything about the east coast.