AndrewAlexanderPrice

Optimizing the Street Grid

November 28, 2013

This post was originally published on the Strong Towns blog on November 27. Since then, it has been referenced over at the Atlantic Cities and Walkable DFW.

Does block size and street width really matter?

The great American street grid;



Some people love it, some people hate it. But you have to admire its simplicity, and it certainly does have some merit;

  1. It is incredibly easy to plan out and survey (everything is an aligned rectangle.)
  2. It is highly connected (there are no dead ends.)
  3. It is very hard to get lost in (especially if you number your streets like in Manhattan.)

Of course, there are disadvantages. Particularly, it can become repetitive and uniform (a strict grid usually does not offer much variation in block sizes and street widths), and it does not accommodate diagonal travel very well. But, I have chosen to focus on the street grid, as it is not the worst street layout there is, and it is very easy to mathematically model and study.

Within the basic street grid, you have some flexibility, namely, in the size of the blocks and the widths of the streets. But, just how much of an effect do these variables have in affecting the walkability and efficient land usage of a city?

To help me in my analysis of the street grid, I developed a simple Street Area Calculator tool. You can input a street width, block size and it will generate some simple statistics about the land usage and walkability of a particular grid. I have used it to generate the statistics and diagrams in this post.

Sample Grids

To study the properties of different street grids, I needed some data to work with. I went on a tour around the United States in Google Maps, and used the measuring tool to measure the block and street sizes in various cities;



The measurements are pretty crude but they are accurate enough to get a general idea of each city's grid. I took measurements for the following cities;

City Street Width Block Size 0.5 Mile Section
Austin, TX 115 ft 345 ft
Little Rock, AR 95 ft 300 ft
Manhattan, NY 58 ft 600 x 250 ft
Portland, OR 60 ft 200 ft
Salt Lake City, UT 130 ft 660 ft
San Francisco, CA 65 ft 420 x 275 ft

Already, we can see some massive differences between the grids. We can tell just by looking at it, that the streets and blocks of Portland (left) and Salt Lake City (right) are vastly different;



Land Usage

One metric we can pull from the grid is how much land is dedicated to the streets versus the actual block. This is an important metric, because essentially, the area used by the block is productive (it can be privately owned, and generates revenue for the city), while the area used by the street places a burden on the city (both due to physically maintaining the surface of the street, and due the lack of land that could potentially be privately owned and generating tax revenue.)

Here are our above cities, sorted by their land usage. From a tax perspective, the less area dedicated to streets, the better;

City Street Width Block Size Street Area Block Area
Manhattan, NY 58 ft 600 x 250 ft 25.99% 74.01%
San Francisco, CA 65 ft 420 x 275 ft 29.96% 70.04%
Salt Lake City, UT 130 ft 660 ft 30.2% 69.8%
Portland, OR 60 ft 200 ft 40.83% 59.17%
Little Rock, AR 95 ft 300 ft 42.32% 57.68%
Austin, TX 115 ft 345 ft 43.75% 56.25%



This is a great metric for measuring how much land has the potential to be put to productive use - for example, I would consider the grid wasteful in Austin, TX where nearly half (43.75%) of land is dedicated to streets, removing the potential for it to be put to productive use.

However, it is not such a great metric for measuring walkability. Walkability is correlated with scale - the closer destinations are, the more destinations that are within walking distance, and the more likely you are to walk. A simple Street:Block ratio does not account for scale differences, and hence, are not good indicators of walkability. This can be observed when comparing Portland's grid to that of Salt Lake City, despite Portland being 'less efficient' by allocating more land to streets (40.83%) than Salt Lake City (30.2%);



Street Frontage

Another metric we can gather from the street grid is how much of the street frontage is within walking distance (street frontage being defined as the perimeter of a block that faces the street.) For example purposes, I will define "walking distance" as refering to half a mile - which may take an average person 10 minutes or so to walk.

An interesting property I have noticed about the street grid is that in most circumstances, 45% to 50% of street frontage of a surrounding square mile is within walking distance, regardless of the scale of the grid. Both of the grids below represent a square mile. Let's assume that we have been dropped into the middle of this grid, with the red areas representing the street frontage that is within a half a mile walk;



You will notice how our walking distance takes the shape of a diamond pattern. This is the result of the grid layout forcing us to use Manhattan distances (we can only walk at 90 degree angles, not diagonally) compared to Euclidean distances (in an organic street layout with diagonal roads, the red area may look more like a circle rather than a diamond.)

Measuring the amount of street frontage within walking distance is an interesting metric, because you could easily correlate street frontage with 'destinations.' A destination is along a street, and takes up street frontage;



So we can reasonably assume that the more street frontage that is within walking distance, the more destinations that are within walking distance, and the more walkable the environment is. (This is a flawed assumption, as the street front may not actually be a destination, and instead a parking lot, or a blank side wall, but it will work as a generalization.) With that in mind, here are our cities sorted by the street frontage that is within a half a mile walk - in this case, larger is better;

City Street Width Block Size Street Frontage Within Half A Mile
Portland, OR 60 ft 200 ft 158400 ft
San Francisco, CA 65 ft 420 x 275 ft 110920 ft
Manhattan, NY 58 ft 600 x 250 ft 106696 ft
Little Rock, AR 95 ft 300 ft 98496 ft
Austin, TX 115 ft 345 ft 83280 ft
Salt Lake City, UT 130 ft 660 ft 56960 ft



This appears to be a much better metric for measuring walkability. Despite Salt Lake City's grid having less land allocated to streets than Portland, Portland's grid has 2.78x the street frontage within walking distance.

Downscaling

The smaller the blocks are, the more there is within walking distance. Let's assume that we have 60 ft wide streets, and we will play with the grid sizes;



The smaller our blocks are, the more they are broken up, and the more street frontage we have. The trade-off is that this greatly increases the amount of land we then dedicate to streets;



The reason behind this is simple - when we break up a block, we need to run a new street through it. Eventually, we reach a point where we are simply wasting space, by allocating more than 50% of our land to streets;

Grid Size Street Frontage Within Half A Mile Street Area
100 ft 207,360 ft 60.94%
200 ft 158,400 ft 40.83%
300 ft 120,960 ft 30.56%
400 ft 101,760 ft 24.39%
500 ft 86,080 ft 20.28%

If we are to downscale our blocks to make our grid more walkable, we also need to downscale our streets, in order to keep the ratio of Street Area:Block Area down. We can have 150 ft blocks and keep our street area down to 22.15%, if we also build 20 ft streets (which would result in 284,800 ft of street frontage being within a half a mile walk - far greater than that of Portland!)

However, when we start talking about 150 ft blocks and 20 ft streets, we begin to get into the realm of traditional cities;



Traditional cities are naturally highly walkable, human-scale environments.

Conclusion

The grid layout is great to study because it is very simple to model mathematically, which allows us to see the effect of different street widths and block sizes on land usage and walkability. The trade-off with the grid layout is choosing between walkability (small, finely grained blocks) or efficiency (large blocks, with very few streets).

In this blog post, I presented a purely mathematical model where all blocks and streets were equal in size. In real life, this is not always the case, and there are plenty of cities that have added their own variation to the grid layout. The best implementation of a street grid that I have seen is Melbourne's. Melbourne has a great mix of wide streets;



And narrow laneways;



In the future, I will discuss other street layouts - such as radial grids, organic streets, cul-de-sacs, and hierarchical systems.



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