Tag Archives: diverging diamond

What Do I Think of the Diverging Diamond Interchange?


By Dom Nozzi

Febuary 2, 2017

Superior, Colorado built a “diverging diamond” interchange that opened in October 2015. It was only the second such interchange to ever be built in Colorado. Traffic engineers are imagessinging its praises throughout the nation. A newspaper article appearing in the February 2, 2017 edition of the Boulder Daily Camera fawned over the fantastic new addition to the region’s transportation system.

I am not joining the engineers or the newspaper in their love affair with the design.

Instead, I find such designs a colossal waste of money – money that could have been used for, say, transit. They are also a colossal waste of land. An entire city could fit inside one of these intersections.

The diverging diamond is a boondoggle for those reasons. But it is also a blunder because they promote increased per capita car travel. Why? In part because they are nearly 51df393d218c6-imageimpossible to cross by foot or bicycle. And in part because in the long run they will further disperse land development in a more sprawling way. Those increased distances will make it increasingly impractical to walk, bicycle or use transit.

Ironically, the major justification for the car-only design is that it briefly reduce intersection congestion, which will initially save a few seconds of motorist time (think of the fiscal irresponsibility of spending millions of public dollars spent to save a few seconds temporarily). Ironic because by artificially inducing more car trips than would have occurred had the diverging diamond not been built, the design will lead to MORE traffic congestion and MORE time delay for motorists in the long term (both at their location and areas in the region).

To hide the embarrassing fact that the millions spent ($14 million in this case) to briefly save a few seconds of time, the publicly proclaimed explanation is that it will improve safety. For the conventional traffic engineer, “improved safety” actually means that motorists can now drive faster and more inattentively with less fear of crashing. No mention is made of the fact that the intersection is much less safe for pedestrians or bicyclists, or that faster, less attentive driving is very dangerous for everyone.

The diverging diamond, therefore, is an excellent example of the century-long failure by conventional traffic engineers to understand induced car trips that are created by (briefly) reducing traffic congestion with these designs. There is a reason, after all, that many researchers repeatedly urge us to understand that it is impossible to build our way out of congestion. It is like loosening your belt to solve obesity.

But wait. There’s more.

Not only is the diverging diamond a boondoggle in the above mentioned ways. It also damages our world by adding more auto emissions into our air (by increasing per capita car trips) and reducing potential tax revenues in the region (by encouraging dispersed rather than compact land use patterns).

Future generations will shake their heads in disbelief over why our generation built these monstrosities.

There is one tiny upside to this overwhelmingly negative idea: it produces future jobs for people hired to remove these mistakes after we regain our senses.

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Filed under Bicycling, Sprawl, Suburbia, Transportation, Urban Design