The recovery of urbanism:

Alley-loaded and on-street parking can easily make new urbanist neighborhoods more than competitive with sprawl in lower-density situations.  In denser center and core zones, though, the parking issue truly comes to the fore. By its basic nature, parking forces uses apart, disrupting the adjacencies that make for effective and efficient urbanism. New Urbanism must provide parking, but not at the cost of coherence.
In planning denser places it is good to keep such ends in mind, and to be aware of basic physical arrangements that comport well with them.  The fundamental goal, so obvious as almost not to bear repeating, is to design neighborhoods that are walkable from the outset and can grow to become even more so in the future.

block structure vital
A plan composed on a system of blocks and interconnected streets is fundamentally important.  Every development should be planned on that basis, even if misguided suburban regulations (such as parking ratios, setbacks, and curb cut limitations) prevent full immediate implementation.
Once the necessity of a block structure is acknowledged, it is possible to judge the efficacy of different block sizes and proportions in their balance of good urbanism and provision of parking. Three practical observations are at the basis of the judgment. First, surface and structured parking have a “natural” dimensional grain of sixty to seventy feet, formed by a two-way travel aisle and two rows of head-in parking stalls.  Second, to obtain good interconnectedness and variety, the blocks composing a neighborhood ought to be no more than a five-minute walk around their perimeters, or about 1,320 feet.  Third, it is best to place parking in the center of blocks, using liner buildings to mask the lots or parking garages.
A rule of thumb, easy to remember, is that two standard-grained bays of parking will be around 120 feet wide; 240 feet of such two-bay layout will normally yield about 100 parking spaces. That is, a double square worth of efficient parking makes a good core for a block. If a 60-foot depth for masking liner buildings (including frontages and any required clearances, but not sidewalks) is allocated around the four sides of such a lot or deck, the resulting overall block is 240 feet by 360 feet. Its perimeter is 1,200 feet, slightly below the empirical maximum.
On the same arrangement, a slightly larger block of 240 feet by 420 feet, possessing the longest allowable perimeter, will yield about 125 center-block parking spaces per level. Adding the on-street spaces around the perimeter, such a block will normally have approximately 185 spaces. Comparing that number with the theoretically available occupiable area of the liner buildings, a rough ratio of 3 spaces per 1000 square feet (per level) has been reached, while maintaining the street wall of buildings intact. The block, at that ratio, is self-sufficient in its parking supply. The ratio 3:1000 may thus be thought of as the threshold above which lies sprawl, and below which lies urbanism and walkability.
Depending upon location and presence of transit, offices “need” something like that ratio of 3:1,000. Retail requires more, often 4:1000 or even 5:1000; merchants will always demand as much as can be mustered, and then some. Residential use, requiring perhaps 1.5 to 2 spaces per household, normally will need less. The goal should be an average of 3:1000 or less. At that level New Urbanism, if its design is otherwise good, will usually be judged more valuable and attractive than conventional suburban development.
Another block variant on the same model is worth noting.  If the central parking field (or deck level) is three bays wide (180’), and if the 1320’ maximum perimeter is still maintained, the resulting block is 300’ by 360’.  The core of parking will contain 150 spaces per level, and there will again be around 60 total on the street faces, for a total of 210 spaces.  Since the “liner” area of this block is of course the same as the 240’ by 360’ block  -- they have the same 1320’ perimeter - the ratio pushes above 3:1000. But a subtle problem is worth noting.  Three-bay structured parking, especially where entirely enclosed, begins to be confusing and offputting.
In preliminary planning it is often usesful to measure a block face in increments corresponding with parking. A rowhouse width corresponding with a single on-street parallel parking space, or two head-in rear spaces, for example — is a first step towards allocating parking use.
No real neighborhood can be composed exclusively of such “perfect” model blocks. Topography, building and frontage types, and Transect location will always inflect layout. But in the craft of urbanism, as with any craft, it is useful to have rules of thumb for comparison and to speed the process.

Copyright © 2007 Andres Duany, Michael Morrissey, and Patrick Pinnell. Libraries, universities, institutions, and businesses may not circulate, hire, print, use as a teaching aid, or reproduce this article and/or images without the prior written permission of the authors.