This is actually the first type of "design" I was interested in. I'd grown up playing SimCity, and was always proud when my cities flourished and turned into self-sufficient entities. And with news of high-speed rail service spreading across the country, I thought this topic should be explored a bit.
Urban design actually shares a lot of characteristics with graphic design. Think about it: a page layout has to flow; all parts need to work with each other to ensure a viewer gets from point A to point B. Similarly, a city, a neighborhood, or a block needs to work in the same way. A citizen needs to get to point B from point A on efficient roads, walkways, or public transit. This is really what design does, as it's been mentioned before: getting a person from point A to point B. It may be in an advertisement or it may be on a street grid. I suggest checking out the urban design link for more information on the topic.