Grid Beats Collector System in Thru-Put Test

Coeur d' Alene, Idaho's US 95 corridor (left) and Portland's Pearl District (right) are contemporary commercial and residential growth patterns. The interconnected street grid of the Pearl District beats the collector system on US 95 in total volume thru-put. The grid is shown to move the same volume of traffic at lower speeds and with a greater degree of accessibility. Drawings are at the same scale.

Simple Do-it-at-Home Test Underscores Need for High Intersection Density

Urbanists, developers, public safety experts, and mobility enthusiasts have long espoused the benefits of an interconnected street grid. Developers prefer the increased number of valuable corner lots, those interested in greater access cite the ability to easily get from point A to point B, urbanists prefer the diversity of experiences and routes, and public safety officials are drawn to the redundancy of the network over one-way-in, one-way-out. These groups often pull in the rare traffic engineer who can speak to the effectiveness of this highly practical mode of organizing our urban fabric with mixed results in convincing DOTs and Public Works Departments of its upside over business-as-usual collector models.

That street networks permit hyper-connectivity and mobility is intuitive enough to grasp. Where the breakdown in the conversation often occurs is explaining to those running overly-complex traffic models that street networks actually have a greater carrying capacity and thru-put than funnelling traffic to common collector routes and onto increasingly massive highways. Conducting the rather simple above test proves that this is so. Using extremes to make a point (Portland's grid is amongst the world's tightest and Coeur d' Alene's highway-oriented retail could be, well, anywhere) the water test shows that a plurality of routes over limited routes increases thru-put while minimizing speed.

The colander represents the street grid. It has a number of differently-sized openings spaced generally evenly. The funnel, on the other hand, is a similar-sized vessel that signifies the collector system, focusing all of the water on a single point. In a side-by-side draining of a gallon of water, the colander easily dispersed the "traffic" volume quicker than the funnel was able to achieve. And it did so at lower individual speeds than the consolidated velocity of the funnel.

This rather unsophisticated model underscores the shortcomings of our transportation establishment that, over the past 50-60 years, has stressed speed over safety, mobility, commerce, and cost. Doing so has turned residents and some businesses to push for street closures and closing off of neighborhoods to choke infiltration of speeding traffic. With every route closed, the degree to which our collector system chokes up increases. This leads to never-ending, expensive widening projects and reducing the viability of non-motorized mobility, diversified development types, and livability of our streets.

By stressing network capacity over facility capacity we can greatly reduce the unnecessary miles traveled, loss of real estate values, and lack of inter-modality that is essential for our environment to be useful to us.