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Low Cost Treatments for Horizontal Curve Safety

CHAPTER 1. INTRODUCTION

SAFETY PROBLEMS AT HORIZONTAL CURVES

In 1998, the American Association of State Highway and Transportation Officials (AASHTO) approved its Strategic Highway Safety Plan¹, which sets a goal of reducing annual highway fatalities by 5,000 to 7,000. To help implement the plan, the National Cooperative Highway Research Program (NCHRP) developed a series of guides State and local agencies can use to identify ways to reduce injuries and fatalities in targeted areas. One target or emphasis area is the problem of crashes at horizontal curves.

A Guide for Reducing Collisions on Horizontal Curves, which is referred to throughout this publication as the Guide, illustrates the problem. The Guide reports that nearly 25 percent of people who die each year on the Nation’s roadways are killed in vehicle crashes at curves. About 75 percent of all fatal crashes occur in rural areas, and more than 70 percent are on two-lane secondary highways, many of which are local roads. Furthermore, the average crash rate for horizontal curves is about three times that of other highway segments. And, 76 percent of the curve-related fatal crashes involve single vehicles leaving the roadway and striking trees, utility poles, rocks, or other fixed objects or overturning. Another 11 percent are head-on crashes, the result of one vehicle drifting into the opposing lane when a driver tries to cut the curve or redirect the vehicle after having run onto the shoulder.

It is because of these dramatic statistics that the Federal Highway Administration (FHWA) has identified Roadway Departure as one of its three program emphasis areas—the other two are Intersection Safety and Pedestrian Safety. One aspect of the Roadway Departure initiative is to develop a series of practical information publications designed for local road agencies. This publication is a result of, and supports Roadway Departure program goals.

PUBLICATION PURPOSE AND SCOPE

The Guide identified 20 strategies as alternative countermeasures—or treatments—to address the specific safety problem at horizontal curves. These strategies share one of two objectives:

Reduce the likelihood of a vehicle leaving its lane and either crossing the roadway centerline or leaving the roadway at a horizontal curve.
Minimize the damaging consequences of a vehicle leaving the roadway at a horizontal curve.

Although the Guide provides information about each strategy, transportation professionals felt that a document providing practical information on where, when, or how to apply a safety treatment or design feature—a resource that includes cost and examples—would be useful to local road agencies. This publication was prepared for this purpose.

There are numerous strategies or treatments agencies can apply to a single horizontal curve or a winding road section to address a safety problem. This publication includes only those engineering treatments that are relatively low cost, as compared to reconstructing the curve or road section to improve the geometric design features, such as degree and length of curve, superelevation, cross section, and shoulders.

The information presented here is concise. To fully cover all the aspects of an individual treatment would require a much larger document that would likely be used less. Rather, this publication provides information specifically relating to local roads and the agencies that manage them. It will help transportation agencies and their crews understand the alternative treatments and how to select and apply them. Where appropriate, and when information was available, this publication provides the following for each treatment:

Description—what it is.
Application Guideline—when to install.
Design—what design elements or materials to use.
Effectiveness—how a treatment can improve safety.
Cost—what it will cost.
Additional sources and contacts.

ABOUT THE MUTCD

Throughout this publication, you will see references to the MUTCD. Shorthand for the Manual on Uniform Traffic Control Devices, the MUTCD defines the standards for all traffic control devices (signs, signals, and pavement markings) road managers install and maintain to help regulate, warn, and guide drivers safely on the Nation’s roadways and streets. The MUTCD is published by the FHWA. All States are required to adopt either the Federal MUTCD or a State MUTCD that is in substantial conformance to the Federal MUTCD. Some States adopt the Federal MUTCD with a State Supplement. State laws regarding traffic control devices should be consulted.

The MUTCD also defines conditions about what, where, and how a device is to be placed or installed. In different chapters of this publication you may see a treatment and the designation that the MUTCD states it shall be used. Shall means something is a standard—a practice or device that is specifically required or mandated—or explicitly prohibited. The MUTCD may designate other treatments as guidance, which tells the road manager that a practice or device is recommended and should be used in typical situations, with modifications allowed for a specific location if an engineering study or engineering judgment indicates the deviation to be appropriate. Finally, the MUTCD provides for options, which are presented as may statements.

To learn more about the MUTCD, visit http://mutcd.fhwa.dot.gov. The site is very easy to use and the Frequently Asked Questions (FAQ) section is very helpful.

INFORMATION IN THIS PUBLICATION

First, a few comments about the publication’s contents:

The treatments discussed are intended to improve the safety of a single curve or a winding section. It is assumed that the agency has identified the location as an existing or potential safety problem. All transportation agencies should have a program for identifying such locations. If it does not, the Guide can help agencies develop such a program.
Some traffic control devices or applications described in this publication do not comply with the MUTCD and are considered "experimental." Any road agency wanting to use a noncompliant device on a public road must request and receive FHWA approval for testing. The MUTCD refers to this as experimentation. MUTCD Section 1A.10 outlines the procedure for experimentation.
Where evaluation information is available, the publication includes estimates of the effectiveness of the treatment in reducing crashes. However, agencies should not expect to obtain these crash reduction values at a specific location, as the actual observed effectiveness of a treatment will vary from site to site.
Several treatments discussed in this publication are signs or other devices placed on supports or posts, which makes them a hazard. The MUTCD states that roadside sign supports in the clear zone shall be breakaway, yielding, or shielded with a longitudinal barrier or crash cushion. For information on breakaway sign supports and the definition of clear zone at http://safety.fhwa.dot.gov/roadway_dept/road_hardware/signsupports.htm.

PUBLICATION ORGANIZATION

The balance of this publication organizes and presents information in the following chapters:

Chapter 2 - Basic Treatments—Devices found in the MUTCD.
Chapter 3 - Enhanced Basic Treatments—Devices considered enhancements to the basic treatments.
Chapter 4 - Other Traffic Control Device Treatments—Treatments not specifically mentioned in the MUTCD, but which are currently used by State and local agencies.
Chapter 5 - Rumble Strips—Overview of rumble strip applications with centerlines, edge lines, shoulders and transversely across the lane.
Chapter 6 - Minor Roadway Improvements—Relatively low-cost treatments that require minor improvements to the roadway or shoulder.
Chapter 7 - Innovative and Experimental Devices—New and innovative devices and other treatments that have not been applied extensively.
Chapter 8 - Maintenance—Simple, effective maintenance practices.

The FHWA encourages readers to use the information presented in this publication to evaluate problems and identify appropriate treatments for problem curve sections. Applying these treatments should help agencies reduce roadway departure crashes and resulting injuries and fatalities. The FHWA also welcomes feedback on experiences with these or other treatments agencies use to solve a safety problem at a horizontal curve. Send us comments and treatment results through the Office of Safety website at http://safety.fhwa.dot.gov.

Image. Cover of NCHRP Report 500 Volume 7: A Guide for Reducing Collisions on Horizontal Curves.