Unlike like Emperor Kuzco, I was actually born with an innate sense of direction.  If you’re like me, and you use the Sun to navigate, you probably appreciate cities with gridded street plans that are oriented in the cardinal directions. If you know that your destination is due west, even if you hit a dead end or two, you’ll be able to get there. However, not all urban planners settled on such a simple layout for road networks. For some developers, topography or water may have gotten in the way. Others may not have appreciated the efficiency of the grid. This visualization assesses those road networks by comparing the relative degree to which they are gridded.

To generate the graphic, I first calculated the azimuth of every road in ten counties (plus one parish and D.C.). I tried to choose consolidated city-counties to keep the focus on urban centers, but for larger counties, I opted not to clip the shapefile to the city boundary. All calculations were made in a sinusoidal map projection using the central longitude of the area of interest. I then graphed the angles on rose diagrams (wind roses) using bins of 5° to show relative distributions for each area. The plots were scaled such that the maximum bar height was the same on each rose. To ensure rotational symmetry in the plots, each azimuth was counted twice: once using the original value and once using the opposite direction (e.g., 35° and 215°). As such, all streets, regardless of one-way or two-way traffic, were considered to be pointing in both directions.

The plots reveal some stark trends. Most of the counties considered do conform to a grid pattern. This is particularly pronounced with Chicago, even though much of Cook County is suburban. Denver, Jacksonville, Houston, and Washington, D.C., also have dominant grid patterns that are oriented in the cardinal directions. While Philadelphia and New York are primarily gridded, their orientations are slightly skewed from the traditional N-E-S-W bearings. Manhattan is particularly interesting because it has a notable imbalance between the number of streets running the width of the land (WNW to ESE) and the length of the land (NNE to SSW). New Orleans and San Francisco express some grid-like forms, but have a nontrivial proportion of roads that are rotated in other directions. Downtown Boston has some gridded streets, but the suburban grids are differently aligned, dampening the expression of a single grid on the rose diagram. Finally, the minimal geographic extents of the grids in Charlotte and Honolulu are completely overwhelmed by the winding roads of the suburbs, resulting in plots that show only slight favoritism for certain street orientations.

If you want to see more detail, a full-resolution version of this graphic can be downloaded here:

https://www.dropbox.com/s/my7y24hrzvhagce/Road_Orientation.png

Data source: http://www.census.gov/cgi-bin/geo/shapefiles2013/main

Script for azimuth calculation: http://www.ian-ko.com/free/free_arcgis.htm

Posted on 23 March, 2014

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