The National Academies

NCHRP 22-17 [Completed]

Recommended Guidelines for Curbs and Curb-Barrier Combinations

  Project Data
Funds: $500,000
Research Agency: Worcester Polytechnic Institute
Principal Investigator: Malcolm Ray
Effective Date: 4/19/2000
Completion Date: 4/18/2004

Background: AASHTO highway design policy discourages the use of curbs on high-speed roadways because of their potential to cause drivers to lose control and crash. Curbs can also cause a laterally skidding vehicle to roll over upon striking the curb, a situation referred to as tripping. In some cases, a barrier is placed in combination with a curb, and inadequate design can result in vehicles vaulting or under-riding the barrier.

While the use of curbs is discouraged on high-speed roadways, they are often required because of restricted right-of-way, drainage considerations, access control, and other curb functions. Highway agencies have typically tried to reduce problems caused by curbs by off-setting the curb from the travel way as far as possible and using different curb shapes. Off-setting the curb is not always possible because of the difficulty with right-of-way acquisition and, in some cases, the risk of detracting from features of historic parkways.

There is a need for nationally recognized guidelines for the design and use of curbs on various types of high-speed roadways. For example, it may be acceptable to use curbs (e.g., mountable curbs or traversable curbs with no vertical reveal) specifically designed to reduce the risks outlined above. Little research has been done on these types of curbs; hence, there is minimal information available on their effect on vehicle tripping or vaulting, especially considering today's vehicle mix. Nationally recognized guidelines are also needed for the use of vertical face curbs such as the AASHTO type "a" curb.

Objective: The objective of this research is to develop design guidelines for using curbs and curb-barrier combinations on roadways with operating speeds greater than 60 kph. The guidelines should take into account the following factors:
  • Curb type, height, configuration, material, vertical reveal, and distance from edge of traveled way.
  • Purposes of curb: aesthetics, hydraulics, delineation, access control, pedestrian refuge, protection of local environment, water quality, and historical preservation.
  • For curb-barrier combinations, barrier type (i.e., flexible, rigid, and semi-rigid), height, configuration, distance from edge of travel way, distance from curb, and end-treatment.
  • Roadside characteristics, including the surface behind curbs such as grass, sidewalks, pavement, or side-slope.
  • Environment.
  • Area characteristics (e.g., suburban or rural).
  • Climatic conditions (e.g., snow and heavy rains).
  • Traffic characteristics, including speed, vehicle mix, and volume.
  • Roadway alignment.
  • Facility type (e.g., parkway, arterial, and freeway).
  • Cross-section (e.g., median, number of lanes, shoulder, and roadside).

Tasks: To accomplish this objective, the following tasks are recommended.

Phase I (1) Conduct a critical review of published literature and ongoing research to identify pertinent design, safety, and operational research on curbs and curb-barrier combinations on high-speed roadways. (2) Conduct a survey of transportation agencies to determine current practice, guidelines, and standards pertaining to the use of curb and curb-barrier combinations on high-speed roadways. The survey should also identify problems experienced by these agencies and solutions developed by the agencies. (3) Investigate the feasibility of using existing crash and geometric databases to characterize the extent and severity of safety problems associated with curbs and curb-barrier combinations on high-speed roadways. (4) Within 6 months of contract award, submit an interim report that discusses the information developed in Tasks 1 through 3. The interim report shall also contain a detailed work plan and budget for completing the remaining tasks. The interim report must present a realistic discussion on the potential for achieving the project's objective within the time and financial constraints of Phase II. The research plan shall provide a 2-month period for review and approval of the interim report. A meeting with the NCHRP panel will be needed to review the interim report and work plan. The research agency shall not begin work on the remaining tasks without NCHRP approval.

Phase II (5) Execute the approved plan developed in Task 4. Provide a draft format and sample of the proposed guidelines to be developed in Task 6 for panel review and approval. (6) Develop draft guidelines that address deficiencies identified. The recommended guidelines should be easily interpreted and expressed in charts, graphics, tables and text suitable for inclusion in AASHTO's Roadside Design Guide and Policy on Geometric Design of Highways and Streets. (7) Submit a final report that documents the entire research effort and includes the recommended guidelines as a stand-alone appendix.

Status: The project has been completed and the final report published.

Product Availability: NCHRP Report 537: Recommended Guidelines for Curb and Curb-Barrier Installations.

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