There is a good reason for the wide street in Chicago or Detroit and that is snow. Or more precisely snow removal.
Those ideas may work well in a mild climate but who wants to trod down a narrow street every day through two feet of snow?
Or through downtown Phoenix when its 115 degrees last week?
Believe it or not, you can remove snow from narrow streets too.
Here's a very common snow removal vehicle: http://image.dieselpowermag.com/f/tech/11090507+w799+h499+cr...
This type of vehicle is used all winter on the narrow paths, sidewalks, and streets at the University of Michigan. Snow levels at U-M are very similar to those in Detroit. I imagine that these vehicles are probably a lot cheaper than the snow plows used for wide streets too.
smaller streets with more people living on them means less snow to move and more people to move it. Also, when you're walking everywhere, snow is FUN rather than an obstacle.
Huge 2-lane roads with 2 foot of snow on them = no school.
Narrow walking streets with 2 foot of snow on them = no problem.
Those ideas may work well in a mild climate but who wants to trod down a narrow street every day through two feet of snow? Or through downtown Phoenix when its 115 degrees last week?