The National Academies

NCHRP 03-61 [Completed]

Communicating Changes in Horizontal Alignment

  Project Data
Funds: $410,627
Research Agency: Michigan State University
Principal Investigator: Richard W. Lyles
Effective Date: 5/7/2001
Completion Date: 8/31/2005

Background: Highway curves tend to be high crash locations. The average crash rate for highway curves is about 3 times the average rate for highway tangents, and the average run-off-the-road crash rate for highway curves is about 4 times that of highway tangents. An understanding of why these crashes are happening may help identify more effective ways of communicating horizontal alignment to road users thereby improving safety.

In lieu of curve-flattening practices or other geometrically based enhancements that are rarely employed on local road systems, traffic control devices offer the most potential for reducing crash rates on horizontal curves. Warning signs (such as turn, curve, winding road, large arrow, and chevrons) with or without advisory speed plaques and a variety of delineation devices (such as wide edge lines, post delineators, raised pavement markers, and rumble strips) are used to communicate changes in horizontal alignment. Nevertheless, such devices are used inconsistently from one jurisdiction to another and even from one location to another within a single jurisdiction, and they are sometimes used improperly. The devices related to horizontal alignment are rarely considered as a system, but merely a collection of individual devices.

Jurisdictions have varying practices regarding the spacing of delineation devices with changes in horizontal alignment; existing devices for roads with multiple changes in horizontal alignment do not provide speed information for each individual alignment change. For example, in a reverse curve with a 40-mph advisory speed, the first curve may be negotiable at 50-mph, leading to a false sense of security and a failure to slow for the second curve that has the 40-mph advisory speed. In addition, a curve with a 30-mph advisory speed may be negotiated at 45-mph leading to distrust for advisory signing. Several recent studies have concluded that the current methodology for selecting advisory speeds is outdated and needs to be reevaluated. Elements such as weather, roadway classification, road user familiarity, and commercial vehicle types may also affect the appropriate advisory speed. A consistent and effective way of determining and communicating horizontal curve information is needed.

Objective: The objective of this project is to develop a methodology to determine and communicate horizontal curve information to road users in a consistent and credible manner.

Tasks: To accomplish the objective the following tasks are envisioned: (1.) Identify design methods, practices, and problems in determining and communicating horizontal curve information to road users. At a minimum, geometric, environmental, driver (human factors), traffic, and vehicle characteristics shall be addressed. Advisory speed determinations as well as curve signing, marking, delineation, and pavement treatments shall also be considered. (2.) Develop and, using available information, do a preliminary assessment of alternative methods and practices to determine and communicate horizontal curve information. (3.) Submit an interim report, within 12 months of the contract effective date, documenting Tasks 1 and 2. The interim report shall also include an updated work plan to evaluate the effectiveness of the most promising alternatives identified in Task 2. (4.) Meet in Washington, D.C. with the NCHRP panel to review the Task 3 Interim Report, including the work plan, approximately 1 month after its submittal. (5.) Evaluate the effectiveness of the alternatives identified in the approved work plan. At a minimum, road-user comprehension and response, cost effectiveness, ease of implementation, consistency, and national acceptance shall be considered. (6.) Develop recommended guidelines for determining advisory speeds and applications to improve the recognition of, and response to, changes in horizontal alignment. (7.) Submit a clear and concise final report that documents the entire research effort and includes a section dedicated to recommended changes to the Manual on Uniform Traffic Control Devices and the Traffic Control Devices Handbook. This section shall be in a format appropriate for submission to one or more of the national committees.

Status: The project has been completed and the final report published as NCHRP Report 559.

Product Availability: NCHRP Report 559

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