It would be ridiculous to suppose there's some kind of "critical distance" involved here, whereby people nearer would always speak of going into the city, and people further away would simply go to the city.
It's just that into more strongly implies going right inside the place (usually, the city or town centre, in this case). It's a metaphor of location, invariably implying a central part of the stated destination.
Going to a place is a metaphor of direction (to = towards, in the direction of). If you're going to London, your intended destination is indeed some part of London, but it might not be a particularly central part.
I live less than 1/2 a mile from my local town centre, but it's a fairly large town, and even in the opposite direction people well over a mile away are still considered to live in this town. And we all speak of going into town when we mean going to the town centre.
Los Angeles is famously "spread out" (the area is almost as big as London, but the population is less than half). People living in outer parts of London commonly speak of going into London to mean going to a much more central part, and I expect Angelenos would be at least as likely to adopt such usages.
So the short answer is you go to a place that's somewhere other than where you started (the preposition represents the direction you'll be going in). You go into a place when your destination is somewhere that could reasonably be described as "inside" that place (the preposition represents your spatial location).
Best Answer
If the question is:
What department are you in?
The answer could be:
Meaning: this answers the question exactly by quoting the name of the department. The asker gets his answer and is now assured that there is indeed a Department in your college with the name "Computer Science".
Or
Meaning: this tells about the stream of learning. There could be multiple departments under this stream. It's a generic answer. The asker gets a general understanding about what your department is all about. But he doesn't yet know what exactly the name of the department is.
or
The role of in/at changes with the context.