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Evaluating the Effectiveness of Geometric Design Measures for Reducing Speeds


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 a majority of crashes occur at or near these locations. 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 (FHWA). 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. The crash modification factors clearinghouse currently has three studies with a CMF for presence of curbs. One study explored raised curb and left-turn channelization in a rural setting. Another developed CMFs for the presence of curbs in suburban settings. The third study explored collision models for multilane segments to examine the safety of curbs. None of these studies directly addressed curb radii at intersections. Additionally, curb radius has been identified multiple times by users on the CMF most wanted list. Thus, by providing thorough research on the impacts of various-sized curb radii in urban settings, this project will produce better guidance for urban intersection design.


Relevant to intersection design, including protected intersections, bulbouts (curb extensions), and on-street parking, research is proposed to:

  1. Quantify the impact of large and small curb radii on pedestrian-automobile, bicycle-automobile, and automobile-automobile crashes.

  2. Understand the impacts of large and small curb radii on specific types of large vehicles.

  3. Develop high quality CMFs for reducing curb radius that apply to pedestrian, bicyclist, and motor vehicle crashes.

  4. 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.

  5. Test if positioning a crosswalk or crossbike nearer to the curbline or farther from the curbline increases safety.


This study is expected to provide guidance on the safety and operational impacts of curb radii on the different users of an intersection. This topic is of specific interest in urban areas.

Related Research:

Levine, K. Curb Radius and Injury Severity at Intersections. Institute of Transportation Studies, UC Berkeley, 2012.

AASHTO, _A Policy on Geometric Design of Highways and Streets _(2011)

Baek,J. and J.E. Hummer (2008). Collision Models for Multilane Highway Segments to Examine the Safety of Curbs. 87th Annual Meeting of the Transportation Research Board, TRB 2008 Annual Meeting CD-ROM. Jonsson, T., Ivan, J.N., and Zhang, C., Crash Prediction Models for Intersections on Rural Multilane Highways: Differences by Collision Type, Transportation Research Record 2019, (2007), pp. 91-98

Lienau, K., "Safety Effect of Barrier Curb on High Speed Suburban MultiLane Highways." TTI-04690-6, McLean, Va., Federal Highway Administration, (1996)


1. Conduct a literature review of studies that detail safety and operational impacts of curb radii, bulbouts and other geometric design measures for speed reduction. Also review policies and procedures followed by various agencies.

  1. Crash Data Analysis – Using crash data from a variety of designs, understand 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. Develop and Publish Curb Radii Recommendations


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


This research is needed as bicycling and walking are increasing in cities nationwide and there is an increased impetus on safety. Providing guidance on the optimal curb radius is critical as cities are designing new infrastructure that seeks to accommodate all modes.

Sponsoring Committee:ANF10, Pedestrians
Research Period:24 - 36 months
Research Priority:High
RNS Developer:Sirisha Kothuri, Robert Schneider and Krista Nordback
Date Posted:01/09/2019
Date Modified:01/11/2019
Index Terms:Geometric design, Speed control, Curbs, Radius, Right turns,
Cosponsoring Committees:ANF20, Bicycle Transportation
Pedestrians and Bicyclists
Operations and Traffic Management
Safety and Human Factors

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