City planners tend to draw circles to show "catchment", the area a particular site serves, such as a bus stop. The circles I've seen recently are normally a quarter mile or so. These circles are quick and easy to draw, but they assume we can fly. It's pretty unrealistic since the street design determines how far somebody can actually go. With cul-de-sacs you just can't walk or bike as far (radially) as you can on a grid, for example.
Take Baton Rouge, a mix of pre-car development and post-car sprawl. Here is a map that compares a 1-mile radius in two parts of Baton Rouge with where you can actually go in a mile if you follow the roads. I went with a mile just to make my life more difficult when constructing the blue lines. If I was smarter, I would have done a quarter mile and done more of these circles.
The first thing you'll notice is that the blue lines in these two circles cover nowhere near the same area. (The second thing you might notice is that neither reaches the full extent of the mile circle.) I'm not really promoting a rectalinear grid as the ultimate design — I think you do have to take the natural terrain into account — but I am definitely saying that there's a price you pay for creating cul-de-sacs all over: it takes longer to leave where you are.
Since I'm new to thinking about cities in this way, I won't be offering corrective advice. There are plenty of smart people out there with brilliant ideas. Check them out.
Thanks to 225 Magazine for linking to this page in September 2012.