Planning: Block Sizes for Larger Lots

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.  I apologize for the primitive graphics.

In my previous post, I provided some calculations to show that rectangular blocks are more taxable than square blocks, and suggested that is why many planners have decided to trade higher taxability for walkability in many cities.  I used unitless dimensions, and I did not get into the details of how specific block lengths and depths might work.  But the details are important, and so I will begin to examine them below.

While I have not decided on an ideal block size, I have picked an overall block size of 300' by 300' with a 60' right-of-way (ROW) on all sides as a reasonable starting point.  (The ROW is split between blocks, so only 30' is taken from each side.)  Those dimensions result in a taxable area of 240' by 240', which happens to be easily divided into many lot widths - 80', 60', 40', 30', 24', 20', and 16'.  With houses on both sides, lot depth is 120' without an alley present.  And 80' is not large by suburban standards, but is a good starting point for exploring walkability and block size.  What does a small block with 80' lots look like?

That is a lot of hard surface area for very few houses.  With a 30' roadway, there would be about 1080 linear feet of sidewalk, or 180' per lot.  If the cost to replace the sidewalk is $200 per linear foot, a major sidewalk replacement program would burden each lot by $36,000.  Of course, that does not happen very often, because estimates of the lifespan of a concrete sidewalk range from about 25 to 80 years.  Roadways have to be repaved more often in most areas, and that can cost about a $1M per lane mile.  With a 300x300 block, repaving could cost about $60,000 per lot.  And most of the paving is doing nothing but providing free parking.  With a 30' roadway and parking on one side, there are about 1.5 spaces per lot of street parking when the side streets are included.  That is in addition to the 2 spaces in each garage and 2 spaces on each driveway.  Clearly, with an 80' lot width, it makes sense to go to larger blocks.

This version looks a lot like a typical moderate-density suburban block, probably because developers are in the business of selling houses, not streets.  (Note that I have included 60' lots as needed in the following calculations because 80' does not divide into some of the lengths evenly.  The general point of the calculations remains.)  In this configuration, each lot has to support about 104' of sidewalk and 109' of street, down from 180' and 200'.  Each lot gets only about 0.9 spaces of on-street parking, or about 0.7 if the 30' cross streets are striped for bike lanes.  Below are the calculations for the two sizes pictured, as well as some others.

	Square	Mildly Rectangular	Seriously Rectangular	Extremely Rectangular	Excessively Rectangular
Gross Length	300	600	900	1200	2400
Gross Depth	300	300	300	300	300
ROW width	60	60	60	60	60
Net Length	240	540	840	1140	2340
Net Depth	240	240	240	240	240
Lots per block	6	14	22	30	60
Sidewalk	180	120	104	96	88
Street	200	129	109	100	90

Developers sometimes build blocks that are around 2400' long, but they are rarely straight, in order to break up the monotony of a street of basically similar houses, and also to conform to local topography when necessary.  In the other direction, the lot depth in a 240' block is sufficient.  A 40' deep house set back 20' from the road would have a 60' deep back yard, or half of the total lot.  That enough room to put at a deck, a pool, and a play set, which are common features of American back yards.

But any neighborhood with lots of these dimensions would not be walkable.  It would be physically safe to circulate on foot, as the traffic density would be low, and the number of street crossings would also be low for the larger blocks.  It could be moderately pleasant if the curb lawns are well-planted with trees.  But it would still be a typical suburban area with little visible activity and nothing to visit nearby.  At the density provided for by 80' lots, it would likely be over a mile from most homes to the nearest shopping center.  Most people would end up driving that distance, leaving walking primarily to children and those who want the health benefits of walking.