Planning: Square or Rectangular Blocks

NB: I believe this topic has been discussed much better many times elsewhere on the net and in print.  I am writing simply to clarify my own thoughts.

In my previous post on planning, I looked briefly at the issue of block size.  Before I pursue that question further, I want to examine why many urban street networks have rectangular blocks, often in a rough 2:1 ratio of length to depth.  Of the grids I mentioned before, Manhattan, the Little Village neighborhood of Chicago, Stockholm, and Turin have blocks that are mostly rectangular.  Square blocks dominate Barcelona, Madrid, Portland, and downtown Chicago.  Many more examples of both tendencies abound.

So if, as I explained in the previous post, smaller block sizes contribute to the quality of walkability, why would planners choose rectangular blocks?  I believe the main driver is infrastructure efficiency, or taxability.  Calculations illustrate the problem quickly.

	Square	Mildly Rectangular	Severely Rectangular	Extremely Rectangular	Large Square
Gross Length	300	600	900	1200	600
Gross Depth	300	300	300	300	600
ROW width	60	60	60	60	60
Net Length	240	540	840	1140	540
Net Depth	240	240	240	240	540
Taxable area	64.0%	72.0%	74.7%	76.0%	81.0%
Increase over previous (baseline for large square)	n/a	12.5%	3.7%	1.8%	26.6%

The increase in the taxable area is most likely why planners choose to make at least part of planned cities rectangular grids instead of Cartesian grids.  Most planners have used a ratio close to the 2:1 proportions shown in the second column, because the returns on higher differences in dimensions decrease.  In addition, if the depth of a rectangular block remains reasonable (note that I did not specify units in the table), unlike with a larger, less walkable square block, lots remain useful without alley access, and can be developed into fee-simple properties up to moderate densities.  The combination of taxability, walkability, and practicality is why so many cities have rectangular grids for at least part of their street network.  However, there are trade-offs in any decision, so planners should continue to respond to local conditions when laying out a particular block or set of blocks.