In 2013, the Los Angeles Department of Transportation rather famously installed a safety improvement project on Rowena Avenue in Silver Lake, with the goal of saving lives by reducing deadly collisions. The “road diet,” as the project was called, shaved the busy commuter surface street from four lanes to three, partially as a response to the death of Ashley Sandeau, who was struck and killed by a vehicle while attempting to cross the street to see her father.

Speed is the No. 1 factor in vehicular homicide. According to the American Automobile Association, the average risk of death for a pedestrian reaches 10% at an impact speed of 23 mph, 50% at 42 mph and 90% at 58 mph. Putting streets on a “diet” slows traffic, making it safer for cyclists and pedestrians and greatly improving the odds that when accidents do occur, the results are far less deadly.

Some nearby residents, however, complained that the new street design — though well-intentioned — increased traffic and decreased safety by diverting drivers onto neighboring residential streets. They organized a much-publicized petition calling for the city to provide an alternative solution to its road diet plan.

Data collected from road diets in other cities across America generally disputes residents’ complaints — despite reducing speed, traffic volume is rarely affected. However, impact studies require time to compile data, something LADOT didn’t have the luxury of as it fended off critiques of its plan. The implications of the controversy are wide-reaching, as L.A.’s citywide “Mobility Plan 2035” calls for the construction of road diets, among other road safety infrastructure, on streets across the city.


And so the debate over whether road diets in L.A. are an important public safety measure or a traffic nightmare-maker has raged on.

Until now.

Since the road diet was installed more than three years ago, LADOT has been collecting data on traffic patterns. An analysis of that data makes it clear that the project has worked as intended: Average speeds dropped from 39 mph to 35 mph, and safety has significantly increased on Rowena, with no effect on overall traffic volume.

To come to this determination, we obtained data collected by LADOT from induction loops — or traffic sensors — embedded in the asphalt throughout Los Angeles County. There are sensors installed at either end of the road diet on Rowena itself, and we examined the data before and after the diet to see its impact on traffic volume.


LADOT’s traffic sensors provided us with estimated vehicle counts about once per minute. We analyzed the average traffic counts on Rowena both before and after the project and found that typical traffic volume was unchanged after the road diet was implemented.

(DataScience )

In addition, publicly available collision data collected by the California Highway Patrol show that motor vehicle crashes dropped after project implementation. Collisions on Rowena were lower in 2013 and 2015 compared with identical periods in 2010 and 2008. In 2008 and 2010, there were six crashes where unsafe speed was a contributing factor. In 2013 and 2015, there were zero crashes involving unsafe speeds. Collisions involving pedestrians and bicyclists also declined after the introduction of the road diet.

Improved safety for all road users, especially pedestrians and bicyclists — the primary goal of the project — has clearly been achieved.


(DataScience )

Measuring the impact on cut-through traffic is more complicated. LADOT does not have sensors installed on Angus, a natural cut-through, making it impossible to directly measure traffic volume. Additionally, Waze and other real-time navigation apps — that proactively reroute motorists onto side streets to avoid congestion — rose in popularity over roughly the same period as the road diet conversion, preventing us from determining the exact cause of any additional cut-through traffic.

Nevertheless, without directly addressing the cut-through issues, we can convincingly conclude that traffic volume is unchanged on Rowena — and that the street is much safer now than it was before the road diet. These results challenge the perception that Los Angeles is too auto-centric for road diets to work. In fact, Los Angeles’ experience with Rowena is consistent with a proven track record of success around the nation.


Concerns about cut-through traffic remain valid, and they can be addressed through engineering solutions that keep the benefits of the road diet intact. Now that the city participates in the Waze “Connected Citizens Program” and can access anonymized data from nearly 2 million users in Los Angeles, future interventions, such as road diets, will be able to be analyzed easily for cut-through impacts, in addition to volume, speed and safety.

Beyond safety, road diets make streets more accessible to pedestrians and bicyclists, bringing us closer to a future Los Angeles unburdened by the polluted, traffic-choked stereotype that exists today. Our analysis of the Rowena project confirms that road diets work — even in the most congested city in America.

Dave Goodsmith and Ben Van Dyke are data scientists at DataScience Inc. You can find them on the DataScience blog and Twitter. Ruslana Dalinina contributed data modeling to this report.

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