Rural, Greenfield areas on the fringe of a major urban area usually grow slowly, until seemingly overnight, they explode with new development.  That’s when congestion overwhelms any through streets that exist, and communities hunt in desperation for opportunities to expand their system.  Prior to congestion, just one home might block an ideal extension.  But one is enough to cause 10-years to pass without funding or political will to protect the rest of the corridor.  Then when necessity is obvious, suddenly it’s 50 homes and businesses, and you may never get what you need.  Thus, huge areas end up saddled with the consequences of poor foresight, and we try to “build our way out” by tearing down whatever we must to make a bad situation “less bad.”

Recognizing the challenges Greenfield areas face as they urbanize, the Institute of Transportation Engineers created a Best Practice recommendation for macro-level network spacing that if adhered to, would minimize congestion on any given facility.  Their spacing guidance is as follows, along with Metro Analytics estimate of typical build-out volumes:

  • 5-mile Freeways: Fully access controlled Freeway, Parkway, or Expressway. 2-4 lanes each direction, with likely traffic volumes of 50,000 to 150,000 at build-out. 50-70 mph speed limits.
  • 2-mile Major Arterials: Either two-way or one-way couplet, usually 2-lanes each direction, with daily traffic of 20,000 to 40,000 at build-out. 35-45 mph speed limits, (with 25-35 preferable across multi-modal activity nodes or centers)
  • 1-mile Minor Arterials: Either two-way or one-way couplet, usually 1 or 2 lanes each direction, with daily volumes of 12,000 to 35,000 at build-out. 30-40 mph speed limits (with 20-30 mph preferable across multi-modal activity nodes or centers)
  • Half-mile Collectors: Usually 1-lane each direction, with daily volumes between 3,000 and 12,000 (2-lane cross section vs. 3-lane cross section, respectively). 20-35 mph typical.
    • To be counted as part of the “macro grid” a collector would both start and end at a similar or higher facility, and should be at least half-mile in length, though ideally far longer (3+ miles).

Applications in Utah

Years ago, Metro Analytics compared Salt Lake County as it was planned to be in 2040 with how it might have looked had it unfolded according to this guidance (above).   Many locations that are now built out are missing both freeway and collector facilities.  Not surprisingly, these are the most congested locations in the state – because every through street has no choice but to serve both regional and local mobility needs.  Also visible are locations with a better than recommended grid, generally in the pre-suburbia traditional grid areas.  In spite of high densities, these areas are also the least congested.

This “Scottish Plaid” comparison of what could have been to what actually is has proven to be a very popular graphic, used in presentations by Envision Utah, UDOT, the MPO’s, and many other agencies.  They use it to motivate greenfield communities to expand the grid where possible, and also to find locations where they’ll need to get creative.

We recently applied the same concept to Utah County, which has much more challenging geography, shown below.

Due to the lake, mountains, and diagonal major freeways and arterials, it is hard to discern much other than “2040’s built areas need more macro-connectivity.”  So in the next graphic, we compared 2040 to a potential build-out, which accounts for all existing freeways, arterials, and collectors, but also plans to preserve corridors to create new freeways, arterials, and collectors for both pre and post-2040 needs.

 

 

Graphics like this are an excellent way to quickly and visually discern how well your reality matches this ideal spacing (and if you’re like most, it won’t be very good).

Also note, this is half-mile spacing, or a thru-street every 2,600 feet!  Historic grids are much tighter than this – with a street of considerable length every 400-700 feet.  Thus this recommendation is only for your macro-grid.  Ideally your micro-grid would be even finer, especially in town centers and other activity centers with considerable emerging multi-modal potential.

Let Us Compare Your Situation to the Ideal

It is very easy to do this, and we are very fast at it.  In just a few hours, we can probably compare your city, county, or region to this ideal, and help you spot problem areas.  We can also do a quick scan of opportunities to create better connectivity, and give you multi-modal project ideas (such as place-making innovative intersections) for addressing locations that cannot realistically increase macro-connectivity.

Author

Mike is president of Metro Analytics and frequent author at Strong Towns. Strengths include Travel Demand Forecasting, Data Visualization, Place-Making Intersections, and Big Picture multi-modal solutions.

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