I wanted a nerdy planning-related poster for my wall (other than the periodic table of city planning), so I made one this week. I scoured Google Earth and measured that quintessentially American grid in about fifty downtowns around the country. Of course, there are variations in block proportions within downtowns, but I tried to pick cities that had more uniformity than average to come up with a single prototype. Here's an image of it, or shoot me an email if you want a poster-quality version. (I personally don't have the web space for it right now).
Exploring these grid proportions messed with my preconceptions. I assumed the more western and newer cities would have larger grids than the more eastern and older cities, but no obvious pattern is discernible to me. Mobile, AL, settled by French colonists in the early 18th century, Tulsa, OK, a 19th century farming town, and Anchorage, AK, a 20th century frontier town, all share the same 300' x 300' internal block (street widths vary a little). What compelled the early settlers of these towns to choose, say, 220' over 440' lengths? I can't say I have any idea right now.
Manhattan is also a curious story. According to Witold Rybczynski, the expanding nation unequivocally chose the 1811 Commissioner's Grid of New York City over L'Enfant's baroque-influenced plan for Washington, DC as the model for new towns. While this is surely true, it begs the question: why are New York's long and skinny blocks not found anywhere else in the country? You would think at least one group of western settlers would seek to emulate their home town of New York more exactly.
I'm leaving aside the interesting value questions around block size. Ever since Jacobs, conventional wisdom has held that smaller blocks are preferable for walkability, but urban designer Fannis Grammenos challenges this somewhat in a recent Planetizen post.