Vision Zero’s War on Arterials Has Been a Failure. It’s Time to Stop Worrying and Embrace the Grid: NMA E-Newsletter #596

By Christopher M. DiPrima, California Member of the NMA

The COVID-19 shelter at home orders have given us the opportunity to evaluate the effects of massive, rapid changes in the use of our transportation system. The results of this giant experiment are trickling in, and they are proving that years of speed-slowing measures on arterials and disinvestment in freeway capacity have increased the risk of vehicle crashes, not decreased it.

Since the dawn of the Vision Zero movement, pedestrian advocates have insisted that slowing vehicle traffic will have a positive effect on safety. Their logic has been that because lower-speed crashes are more survivable than higher-speed crashes, and because humans are inherently incapable of driving safely, the best course of action is to slow everyone down as much as possible. Coupled with a simplistic and incomplete understanding of induced demand (the idea that providing more capacity creates more traffic), these advocates believe that by reducing capacity, they can take sufficient vehicles off the road to improve safety.

This effort has taken several forms, including removing travel lanes, restricting turns, shortening traffic signal cycle times, and removing intersection level of service from development criteria reviews.

The natural result of slowing down traffic on arterials is that motorists have looked for shortcuts, usually on parallel neighborhood streets, which were never designed to be thoroughfares. Residents of these parallel streets notice the increase in traffic and complain to their governments, often leading to the installation of speed humps, chicanes, stop signs, and other traffic-slowing measures.

With shelter at home orders drastically reducing traffic, the need for these shortcuts disappeared overnight. Why cut through neighborhoods when the freeways and arterial streets are flowing well? Indeed, one report by Joint Venture Silicon Valley’s Institute for Regional Studies found that while per-capita freeway vehicle miles traveled (VMT) dropped 41 percent due to the shelter orders, all-road VMT declined by 75 percent or more. (The report, “Silicon Valley COVID-19 Impacts: Transportation, Emissions & Air Quality,” can be downloaded from here.)

This substitution effect, where more traffic returns to freeways because they offer a speed advantage, is one of the reasons why these freeways were built in the first place. As early as the 1950s, studies like the Detroit Metropolitan Area Transportation Study and the Chicago Area Transportation Study showed that the introduction of freeways to a road network tended to increase traffic on perpendicular routes but reduce traffic on parallel ones. For the Peninsula, this means that increasing freeway capacity on US-101 and I-280 should decrease traffic on local north-south streets like El Camino Real – exactly what is happening now that the freeways once again offer a meaningful travel time advantage over parallel surface streets.

Because freeways are statistically the safest roads, this should lead to a drop in crashes and deaths per VMT. Indeed, despite the invented moral panic surrounding “super-speeders,” this has confirmed itself as Bay Area injury crashes per vehicle mile traveled have declined by almost 50 percent, and unsafe speed crashes have declined 79 percent year-over-year. In other words, despite traveling faster, we are getting into fewer crashes – even accounting for the reduction in travel miles – because we are driving on safer roads.

While this may seem counterintuitive, it should be obvious to any experienced motorist. As the Vision Zero lobby has constricted traffic flow, it has turned arterials into a Hobbesian war of all against all. It does this principally through two means:

  1. It makes road users aware that they have limited time to make it through each intersection. This makes them take more risks to ensure that they can clear intersections.
  2. As intersection level of service declines, and especially as it becomes uneven, it becomes more advantageous for motorists to avoid certain intersections, or to approach them from a different direction. This, in turn, increases the number of turning movements required. As the number of turns increases, the number of vehicle-vehicle, vehicle-bicycle, and vehicle-pedestrian conflict points, and combined with the increase in risk tolerance discussed above, the decline in level of service should lead directly to an increase in crashes.

The answer to this problem is simple and has been known to engineers for decades. In the interest of safety, Vision Zero must abandon its war on arterial streets. Those streets should be designed to increase vehicle throughput, thus reducing diversion onto other streets. They should encourage straight-through driving, rather than the multitude of turns now required to optimize travel time. Far from making the system more dangerous, it will make it safer.

This doesn’t mean that all of the safety improvements that we have added to arterials should go away; to the contrary, adding turning pockets, daylighting crosswalks, adding crossing beacons, and hardening medians are just a few of the sound principles which improve safety without decreasing capacity and causing the attendant negative side effects.

Rather than blindly applying traffic-slowing measures in neighborhoods, we should identify shortcutting as a symptom of a failure of our arterial street system and make real fixes to those facilities. And yes, in some cases, that will mean expanding streets so that more people can drive on them. But the result will be neighborhood streets much more like the ones that Vision Zero advocates lionize: streets that serve all users at the pace of life, instead of hybrids which serve poorly both neighborhood residents and through motorists.


Editor’s Note: This is typically where we would add a “The opinions expressed here are the author’s own,” but we find ourselves in full agreement with Mr. DiPrima.

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