The effects of left-turn traffic-calming treatments on conflicts and speeds in Washington, DC

Last updated: 12-24-2020

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The effects of left-turn traffic-calming treatments on conflicts and speeds in Washington, DC

Volume 75 , December 2020, Pages 233-240
The effects of left-turn traffic-calming treatments on conflicts and speeds in Washington, DC
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Abstract
Introduction: Left-turning vehicles pose considerable safety risks to pedestrians at intersections. Left-turn traffic-calming treatments are designed to slow left-turn traffic. This study examined the effects of one type of left-turn calming, the hardened-centerline treatment, on the numbers of conflicts between left-turning vehicles and pedestrians and left-turn speeds in Washington, DC. Method: Numbers of conflicts between left-turning vehicles and pedestrians, as well as left-turn speeds, were collected at selected intersections in Washington, DC, where the hardened centerline was installed, as well as at control intersections in the city where no treatment was installed, before and after installation. Poisson regression evaluated the change in numbers of conflicts associated with the hardened-centerline treatment. The effect of the treatment on left-turn speeds was estimated by a log-linear regression model, and the effect on the odds of left-turning vehicles exceeding 15 mph was estimated by a logistic regression model. Results: The treatment was associated with a 70.5% reduction in conflicts between left-turning vehicles and pedestrians, a 9.8% reduction in mean left-turn speeds, and a 67.1% reduction in the odds of left-turning vehicles exceeding 15 mph. All the reductions were statistically significant. Conclusions: The study demonstrates that the hardened-centerline treatment can reduce conflicts between left-turning vehicles and pedestrians, and slow down left-turn traffic at intersections. Practical applications: The treatment should be added to the toolbox for communities looking to improve pedestrian safety at intersections.
Introduction
Pedestrians are among the most vulnerable road users, and they represent a considerable portion of motor-vehicle crash injuries and fatalities. In 2018, a total of 6,283 pedestrians were killed in motor-vehicle crashes in the United States, accounting for 17% of total crash deaths. Pedestrian fatalities have increased 53% since reaching their lowest point in 2009 (Insurance Institute for Highway Safety [IIHS], 2020).
Intersections represent one of the most complex traffic situations with multiple approaches and crossing movements by motorists, pedestrians, and bicyclists. In 2018, 55% of crashes involving pedestrians occurred at intersections, resulting in more than 6,700 pedestrians with serious injuries and 1,562 pedestrian deaths (IIHS, 2020). One of the most common conflicts at signalized intersections is between vehicles permissively turning left and pedestrians crossing during the concurrent pedestrian signal phase (Zegeer, Nabors, & Lagerwey, 2013). The resulting crashes accounted for 30% of all pedestrian-involved crashes at intersections, killing a total of 192 pedestrians in 2018 (IIHS, 2020).
Left turns are complicated maneuvers and pose considerable safety risks to pedestrians at intersections. Drivers turning left must monitor oncoming traffic and pedestrians to find gaps, while vehicles behind are waiting. The high workload could result in drivers skipping certain tasks or proceeding improperly (Richard, Campbell, & Brown, 2006). Left turns usually occur at higher speeds than right turns (Roudsari, Kaufman, & Koepsell, 2006). The conflict zone in the crosswalk between pedestrians and left-turning vehicles is larger than between pedestrians and right-turning traffic, due to some vehicles cutting across the center line during left turns. There are also visibility issues with left turns, since a driver’s view of pedestrians in the crosswalk could be obscured by the vehicle’s A-pillar during left turns (Reed, 2008).
To address the pedestrian safety problems involving left-turning vehicles at intersections, countermeasures have been developed and shown effective, such as leading pedestrian intervals, exclusive pedestrian phases, and protected left-turn phases (Fayish and Gross, 2010, Mead et al., 2014). Alternative intersection designs, such as roundabouts and median U-turn intersections, eliminate potential pedestrian exposure to left-turning vehicles (Reid et al., 2014, Rodegerdts et al., 2010). At roundabouts, vehicles travel in the same direction around center islands. Median U-turn intersections relocate left turns from the main intersection to U-turn openings in the median beyond the main intersection.
Left-turn traffic-calming treatments have been recently installed in a growing number of cities. These treatments are designed to slow left-turn traffic by forcing vehicles to make a more 90-degree left turn, instead of cutting through a crosswalk at a slight angle, reducing the size of the conflict zone for crossing pedestrians (Fig. 1). New York City is the leader of left-turn traffic-calming efforts, and these treatments have been installed at more than 300 intersections during 2016–2018 in the city. At intersections where the treatments were installed, the observed left-turn speeds were lower than before installation (New York City Department of Transportation, 2019).
Washington, CD, as part of its Vision Zero efforts, started installing left-turn traffic-calming treatments in 2018 and aims to have the treatments installed at a total of 85 intersections by the end of 2020. The effects of left-turn traffic calming have not yet been rigorously studied. One type of the treatments, known as the hardened centerline, consists of rubber speed humps and bollards installed on “Qwick Kurb” anchored to the yellow center line of the intersection leg that vehicles are turning into (Fig. 2). It is installed where a one-way or two-way street meets a two-way street. The objective of the current study was to evaluate the effects of the hardened-centerline treatment on the numbers of conflicts between left-turning vehicles and pedestrians, as well as on left-turn speeds, in Washington, CD.
Figures
Left-turning vehicle trajectories before and after the hardened-centerline treatment.
The hardened-centerline treatment in Washington, DC.
Map of treatment and control intersections in Washington, DC.
a. Hardened-centerline treatment with rubber speed humps extended into the intersection. b. Hardened-centerline treatment with rubber speed humps extended to the crosswalk. c. Hardened-centerline trea...
Section snippets
Method
Numbers of conflicts between left-turning vehicles and pedestrians crossing parallel to the vehicles prior to left turns, as well as left-turn speeds, were collected at selected intersections in Washington, DC, where the treatment was installed and at control sites in the city where no treatment was installed, both before and after the installation.
Effects on conflicts
Table 2 summarizes the mean, minimum, and maximum numbers of observed left-turning vehicles, pedestrians, and conflicts per site by study group and study period. At one treatment and one control site, no conflicts were observed during both the before and after periods. From the before to after period, the average number of conflicts per site declined 69.6% (6.9 vs. 2.1) at treatment sites, but remained unchanged (1.4) at control sites.
Table 3 provides estimated results of the Poisson regression
Discussion
The recent upward trend in pedestrian fatalities has prompted many cities to make efforts to improve safety for all road users. Frequent pedestrian activity occurs at intersections, and left turns are dangerous vehicle movements to pedestrians. In Washington, DC, the hardened-centerline treatment was associated with significant reductions in conflicts between left-turning vehicles and pedestrians, mean left-turn speeds, and odds of left-turning vehicles exceeding 15 mph. This is the first known
Acknowledgements
The authors wish to thank Wasim Raja and Clarence Dickerson from the District Department of Transportation for providing treatment installation information, and Arban Vigani from New York City Department of Transportation for providing information on pocket radars. This work was supported by the Insurance Institute for Highway Safety.
Wen Hu is a Senior Research Transportation Engineer at the Insurance Institute for Highway Safety. She holds her Ph.D. in Civil Engineering from the Pennsylvania State University. She joined the Institute in 2010 and has been involved in such topics as speeding, automated enforcement, and roundabouts.
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