The first thought I have on Old Urbanism is about street width. Old Urbanists--particularly Nathan Lewis and Charlie Gardner--argue that our streets should be narrower. A lot narrower.
I have no quarrel with this, in principle. There really can't be much of an argument that postwar streets are excessively wide. When I learned in Land Use Planning 101 that the 1960 Philadelphia Subdivision Code mandated streets of at least fifty feet wide, I was shocked; in the lived environment of urban Philadelphia, 50 feet (three traffic lanes and two sidewalks) is the widest a street feels comfortable at.With the exception of the grand boulevards or proto-grand-boulevards (for example, Broad, Market, JFK, Washington, Spring Garden, Girard, Delaware, the Ben Franklin Parkway), no street in Philadelphia's urban core is ever historically* more than 50 feet wide, and the traditional driving of alleyways through the middle of blocks results in a network of ever-narrower streets and mews. Even so, since Philadelphia's primary grid is built up of 50-foot-wide streets (arterial streets, to the Old Urbanists), the mix here is closer to 49-49-2, in terms of narrow streets, arterial streets, and grand boulevards. This hasn't impinged Philadelphia's urbanism a bit.
Secondly, traffic density can also be a determinant of walkability or human-centrism. The primary Chestnut and Walnut Street shopping core is aligned along a pair of 50-foot-wide streets with two ten-foot sidewalks, two traffic lanes, and a parking lane**, but pedestrians and automobiles are segregated due to no more than the sheer force of traffic. Midcentury planners attempted to pedestrianize such streets--Chestnut was pedestrianized for a while--to disastrous results.
That said, I think that no more than the barest handful of streets need be more than 50 (or at the very outside, a very dense 60, like Ridge Avenue here) feet in width. Beyond this, they split rather than knit. Different neighborhoods also require different street widths to feel comfortable. Neighborhoods built on the picturesque-suburban model*** (such as Wynnefield) feel most comfortable with 50-foot roads; more urban neighborhoods are more comfortable with roads that run between 15-20 ft. and 50 ft., with each city having its own idiosyncratic alchemy of the two: this subtle alchemy helps creates a sense of macro-place: we are here and nowhere else.
The second thought I have--since streets are so wide and setbacks so deep--is that we can narrow our streets, and cities with excessively wide streets can make a pile of money by literally selling off excessive street width, in addition to other densification strategies--granny flats along the alley, carving excessively-deep front yards into new buildable parcels, and flat-out per-parcel redevelopment. In places like Detroit, where, for example, the widest streets can reach 200 ft., you can get whole new blocks.****
There are other ways to ameliorate excessively wide streets. Transforming their centers into various types of green space (hedges, swales, and even alamedas) is a good way to make them less excessively autocentric; an alameda is--additionally--a good way to provide bike facilities in underutilized street space. In a handful of instances, going farther is also an option: a trolleyway down the center of this greened space in the center of the street, maybe?
* Some streets have been widened since. Some the numbered streets accessing the postwar office core of Market West have been widened to 60 or as much as 80(!) feet.
** I do dispute on-street parking a bit...although not by much. The occasional on-street parked car helps control speeding and defines the carriageway, for autos, and effectively becomes a piece of street furniture presenting a lived feeling in the neighborhood for pedestrians: good examples are this narrow (30 ft., I think) street in Manayunk and this street in Wynnefield.
*** Picturesque suburbia is heavenly. Post-picturesque-suburbia becomes ever more hellish.
**** This is not to say that Detroit has a market that can take this additional density, however.