Road diets: Add capacity and access for all

March 20, 2013



Slow gets more of us there faster

When the proverbial ‘they’ say a road diet or speed reduction will cause congestion or some other end-of-the-world-malady, remember this graph posted in a Better Cities post recently (BC). The graph depicts how traffic travelling 25-mph and under may actually increase the number of cars that travel per hour through an area.

VPHPL = Vehicles Per Hour Per Lane

Increased traffic for all users

Also to be considered, but that isn’t depicted in the graph above, is the impact a slower, more accessible street will have in increasing traffic volumes for people on foot and on bike. Increasing the safety, comfort and appeal of self-propelled transportation through an area has additional volume impacts that is seldom measured. But, the more an area can facilitate parking once and walking between businesses or all together choosing to reach the area by transit, foot or bike, those trips also represent an increase in traffic that likely wasn’t happening before.

Of course, there are other considerations, chief among them, as the author, Chuck Marohn points out, is intersection design, which is the key to traffic flow and capacity. If you have a botched-up intersection, the best street design leading into it is for all for not. To gain increased efficiency with slower speeds, the intersection, and the next, and the next are critical–keeping the intersections flowing–smoothly and slowly–is the goal.

Corridor diets

As Traverse City considers road diets on 8th and Garfield Ave (Corridor Study), two streets with higher than posted speeds, reducing lanes need to be understood as way to help ameliorate a major community concern–the discomfort and safety concerns caused by high speeds. The Federal Highway Administration lists road diets as proven safety countermeasure (FHWA) because road diets, in part, create a scenario where one car going the speed limit (25-mph) or below sets the pace for everyone behind them. With 4-lane stroads, like Traverse City’s worse streets (yes, you 8th Street!), the effective and perceived speed is currently set by the most aggressive drivers.

This phenomena was mentioned on MyWHaT back in 2010 concerning Traverse City’s Division St. (here and here ). Unfortunately, a road conversion won’t happen on Division St. anytime soon. MDOT is one of the most cautious DOTs in the country regarding road diets and traffic volumes (MI-GOV). MDOT doesn’t (or seldom does) recommend these road conversions with volumes over 10,000 ATD (Average Daily Traffic) or peak hour traffic counts over 1000 vehicles (I once heard MDOT’s minimum as 15,000 ATD, but I couldn’t find reference this morning). Other DOT minimums are higher; a minimum of 25,000-ATD (FHWA) is often referenced.

That said, there is flexibility to experiment on streets with larger volumes when the context is a mixed use, urban, under-performing corridor–and there exists the political will to do so. Even more so when those streets are local streets, under local control. Garfield Avenue (under 20,000-ATD) and 8th St.’s 1/2 mile four lane section (under 25,00023,000-ATD) pose challenges for road diets, but they are not stretches–it can be done and there have been successful road diets on much busier streets.*

In the future, we’ll write about the primary reason road diets need to be considered in Traverse City. It isn’t for solving traffic problems, it’s to help release the economic potential and value that these two corridors contain. Changing how the public streets running through our neighborhoods behave is critical to a stronger and more resilient localized community and that is something worth counting.

* Traffic counts on Traverse City Streets need to be understood as snapshots for one time period–typically one day–in one spot. The City admittedly lacks the resources for comprehensive data gathering and analysis. In addition, traffic counts remain only one piece of the equation. 


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Disclaimer: Opinions expressed here are that of the author and do not represent the opinions of writers previously published here or any of the organizations, committees, commissions or other affiliation the authors may belong to, unless so stated. 

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