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Effectiveness of Intersection Turning Speed Reduction Strategies


Urban areas have a mix of modes (pedestrians, bicycles, automobiles, and transit) and intersection designs (e.g., number of through lanes, turning lanes, signalization, crosswalk placement, curb radius). Intersections are important for multimodal safety improvements because intersection crashes constitute a significant portion of total crashes nationwide that amount to approximately 44 percent of all reported crashes.

A common crash type for pedestrians and bicyclists is impact from a right-turning vehicle at an intersection, which can be exacerbated by a wide curb radius, which is often associated with high-speed turns. Right-turn lane designs include a shared lane with an island, shared lane with an island and a dedicated downstream lane, right-turn lane with an island, right-turn lane with an island and dedicated downstream lane, and shared lane. The FHWA Pedestrian Safety Guide and Countermeasure Selection System recommends curb radius reduction as a strategy to improve pedestrian visibility, improve sight distance between motorists and pedestrians, reduce the crossing distance for pedestrians, and reduce turning speeds for vehicles. However, smaller curb radii may have a negative impact on turning for trucks, buses, and other large vehicles.

There are no extensive studies that quantify the impacts of curb radius on pedestrian, bicyclist, and motorist crashes, and there are no extensive studies that provide a thorough discussion of the tradeoffs of curb radius among types of intersection users and vehicles in different contexts. Additionally, curb radius has been identified multiple times by users on the CMF most wanted list. The crash modification factor (CMF) clearinghouse currently has three studies with a CMF for presence of curbs: one that explored raised curb and left-turn channelization in a rural setting, one that developed CMFs for the presence of curbs in suburban settings, and one that explored collision models for multilane segments to examine the safety impact of curbs. There is evidence that curb radius reductions are effective, however no CMFs are available for urban settings. Left turn crashes also lead to fatalities and serious injuries for pedestrians and bicyclists. During 2016-2018, NYDOT installed left turn traffic calming treatments (hardened centerline, slow turn wedges) at 330 locations and found a 20% reduction of pedestrian crashes and 19.9% decrease in median left turn speeds. Yet, more research is needed to fully study the impacts of these treatments at intersections. By providing thorough research on the impacts of various-sized curb radii and left turn speed calming treatments in urban settings, this project will produce better guidance for urban intersection design.


Relevant to intersection design, including protected intersections, bulbouts (curb extensions), on-street bicycle markings, and on-street parking, research is proposed to quantify the impact of large and small curb radii and left-turn calming treatments on pedestrian-automobile and bicycle-automobile crashes. The research should also develop high quality CMFs for reducing curb radius and left turn calming treatments that apply to pedestrian, bicyclist, and motor vehicle crashes and understand how lateral offset distance (the distance between a motor vehicle lane and a parallel crosswalk or crossbike) affects yielding/ turn speed, bike and pedestrian comfort, and bike and pedestrian safety at intersections. Finally, the research should test if positioning a crosswalk or crossbike nearer to the curbline or farther from the curbline increases safety.


There is an increasing need to improve safety of the alternative modes and therefore this study is expected to provide guidance on the safety and operational impacts of intersection turning speed reduction treatments on the different users of an intersection. This topic is of specific interest in urban areas.

Related Research:

This research is needed as bicycling and walking are increasing in cities nationwide and there is an increased impetus on safety. While a handful of studies have investigated the impacts of certain treatments (Baek and Hummer, 2008, Levine, 2012, Lienau, 2016, NYCDOT, 2016) there is a need for a larger study to comprehensively study all the intersection speed reduction treatments and provide guidance for cities as they design new infrastructure that seeks to accommodate all modes.


1. Conduct a literature review of studies that detail safety and operational impacts of curb radii, bulb outs and other geometric design measures including left turn calming treatments for speed reduction. Also review policies and procedures followed by various agencies.

  1. Crash Data Analysis – Using crash data from a variety of designs, investigate the safety impacts and develop CMFs.

  2. Simulator Based Experiment – Design a simulator-based experiment to study the various curb radii to determine appropriate guidance for all user types.

  3. Provide guidance to practitioners on the tradeoffs associated with each treatment along with best practices on when to use each treatment.


Traffic and geometric design professionals responsible for facility design and planning.

Sponsoring Committee:ACH10, Pedestrians
Research Period:24 - 36 months
Research Priority:High
RNS Developer:Sirisha Kothuri, Subasish Das, Krista Nordback, Robert Schneider
Source Info:AASHTO, A Policy on Geometric Design of Highways and Streets, 2011.
Baek, J. and Hummer, J. E. Collision Models for Multilane Highway Segments to Examine the Safety of Curbs. Presented at the 87th Annual Meeting of the Transportation Research Board, Washington D.C., 2008.
Fitzpatrick, K., Das, S., and Contreras, A. Is Age a Factor in Crashes at Channelized Right-Turn Lanes? An Exploration of Potential Relationships. ATLAS-2016-14, 2016.
Jonsson, T., Ivan, J. N., and Zhang, C. Crash Prediction Models for Intersections on Rural Multilane Highways: Differences by Collision Type. Transportation Research Record, Vol. 2019, 2007, pp. 91-98.
Levine, K. Curb Radius and Injury Severity at Intersections. Institute of Transportation Studies, UC Berkeley, 2012.
Lienau, K. Safety Effect of Barrier Curb on High Speed Suburban MultiLane Highways. TTI-04690-6, McLean, Va., Federal Highway Administration, 1996.
New York City DOT. Don’t Cut Corners. Left Turn Pedestrian and Bicyclist Crash Study, 2016.
Date Posted:01/03/2020
Date Modified:02/21/2020
Index Terms:Intersections, Curbs, Left turns, Right turns, Turning radius, Highway safety, Pedestrian safety,
Cosponsoring Committees: 
Pedestrians and Bicyclists
Safety and Human Factors

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