Blocks should have sides greater than 200 feet and less than 600 feet, with a perimeter less than 1,800 feet.
Perhaps 80% of good urban form is embedded in this one rule; no other metric is as powerful as this one. Many of the desired aspects of urbanism, including walkability, adequate street frontage, and adaptable land subdivision, are a distinct function of block size. If one were to implement only one rule, this would be it.
The image below compares the median-sized blocks of various neighborhoods. At first glance, some of the blocks are a little more angular than others, some have alleys while some don’t, but overall they are remarkably similar. They are all relatively the same size, relatively the same shape. Notably, in some cases they are indistinguishable: for example, the Garden District of New Orleans and downtown Madison, Wisconsin, utilize essentially the same block. And Charleston and Vienna are barely distinguishable from one another. A worldwide pattern is already evident here.
Now let’s compare these blocks in another way. We can reduce median block sizes to a point on a graph (refer to the image below), with the average short block side for each neighborhood falling along the x-axis and the average long block side falling along the y-axis. If block size was random or did not factor in determining great cities, we would expect to see these points dispersed haphazardly, but amazingly they huddle together in one area. This is not an outcome of the universe telling us how big a block should be; instead, it is a product of millions of decisions made by people and planners over the centuries, fine-tuning the dimensions of urban form. The consensus is astonishing.
Jane Jacobs, the venerable New York journalist and observant urbanist, stated in her famous book The Death and Life of Great American Cities:
“Frequent streets and short blocks are valuable because of the fabric of intricate cross-use that they permit among the user of a city neighborhood. Frequent streets are not an end in themselves. They are a means toward an end… Frequent streets are effective in helping to generate diversity only because of the way they perform. The means by which they work (attracting mixtures of users along them) and the results they can help accomplish (the growth of diversity) are inextricably related. The relationship is reciprocal.”
Our analysis above reveals the numbers behind Jacobs’s observations. Our study of great places shows that on average their blocks fall somewhere between 200 feet and 600 feet. As one additional constraint, their perimeters are bounded by about 1,800 feet (e.g., a square block 600 feet on each side has a perimeter of 2,400 feet which falls outside the range). What is incredible about this is that our study incorporates places not only all over the world but places that were built at various times, from decades or even thousands of years ago. Yet the data show us that the range we are talking about isn’t 0 to 10,000 feet, or 10 to 5,000 feet; it is a tight 200 to 600 feet.
If we break it down between each side of a block the variances get even tighter. The figure above also tells us something about block orientation. The average short side range is only 200 feet to 400 feet; the average long side range is larger: 300 feet to 700 feet. The reason for this is that typically the short side is a function of a lot’s depth. Because blocks are typically composed of two lot tiers and because lot depths typically fall in the 100 ft to 200 ft range, this narrows the overall range of block width. Block length, however, is a function of lot width and can change simply by appending more lots side by side. In this case, the block length can easily expand without sacrificing lot efficiency. This is why block length falls within a higher range.
It is a common misconception to assume that small blocks imply high population density and big blocks imply low population density.[^This likely stems from the fact that many cities built at the turn of the century incorporated small blocks, and over time development has naturally been concentrated there of decades or centuries.] As it turns out, block size has little relationship if any to population. In the analysis shown below, the populations span a wide spectrum, ranging from 488 people in Paragonah, Utah, to 2.2 million people in Paris, France. However, amongst all the cities shown, their block sizes are in an incredibly tight range, with a relative standard deviation[^Recall from your statistic class that relative standard deviation is a standard deviation normalized about the mean. In other words, it is a measure of variance that allows you to compare completely different things: in this case, population and block size.] of only 28%. Essentially, what is being reflected here is a universal urban form: all these cities represent the spectrum of everything ever built on the planet, and yet they’re all operating within the same relative block size.
Finally, we can see how differently a 400-foot square block can be used (refer to the image below). While all the blocks represented are the same size and shape, they have each come to accommodate an incredible range of land uses. One is a farming block in Arizona, the other a small town in Utah, and the other the thriving downtown of Chicago. Same block, different use.
This discussion of block size also leads to one on lot size. The dimensional adaptability of blocks continues down to the dimensional adaptability of lots. For example, a standard 60-foot by 120-foot lot is a surprisingly versatile unit of land. That lot could accommodate everything from a house to a parking lot to a skyscraper. While the heights and land use intensities vary, the critical point is that all these uses essentially occupy the same foot print. In plan view, a house with a yard can take up just as much space as a 40-story skyscraper.
Lot widths in multiples of 12 are ideal, dimensionally, because 1) they can accommodate various land uses, 2) standard building dimensions (which are based on units of 12) and 3) superior mathematical fungibility.[^Refer back to the discussion of number theory in the introduction.] Furthermore, lots work best when the narrow side faces the right-of-way. Some reasons for this include:
- It is more economical for a lot owner to own a smaller portion of street a right-of-way than otherwise.
- Smaller frontage units allow more lot owners to face the street per unit length which translates into more opportunities for a vibrant mix and proximity of land uses.
- Smaller frontages also allow for more access points (shop fronts, front doors, etc) which further contributes to vitality.
- Building layouts tend to favor elongated rectangles over squares for various reasons including the limits of natural light and the proportions of individual rooms (not every room in a house or office needs to be a square).
Let’s now leave the discussion of small blocks and consider what happens when blocks are too big. In Salt Lake City, Utah, the blocks are 660 feet on a side with a perimeter of 2,640 feet. This clearly does not pass our Rule #1. This block depth implies a lot depth of 330 feet. As we have just seen with the discussion about lot size, there are very few things that we build in this world that require a 330-foot lot depth. Because of that, in Salt Lake City what has happened over time is that property owners along the edge have sold the back half of their lots. This has resulted in an inner block of development, but this interior development does not benefit from the exposure that a street provides; consequently, they do not contribute to the vitality of Salt Lake City’s streets.
The diagram below further illustrates the absurdity of Salt Lake City’s block size.
Recall that a block is defined as an area of private property surrounded on all sides by public rights-of-way. Let’s apply that definition to the blocks of Alpharetta in suburban Atlanta, Georgia. The urban form of Alpharetta possesses innumerable cul-de-sacs, and by definition cul-de-sacs do not connect through, they stop and avoid connection. This results in block sizes of enormous scale which looks all the more ridiculous even when compared to the over-sized blocks of Salt Lake City (see image below). Given the weight that block sizes have in urbanism and based on these observations alone, it is safe to say that Alpharetta will never possess the qualities of a walkable, vibrant, mixed use community; it simply does not have the urban form to support it.
