Horizontal Curves

Introduction

The AASHTO Strategic Highway Safety Plan identified 22 goals that need to be pursued to achieve a significant reduction in highway crash fatalities. Two of the goals within the plan include Keeping Vehicles on the Roadway (Goal 15) and Minimizing the Consequences of Leaving the Road (Goal 16). Several emphasis areas have evolved from these two goals: run-off-road (ROR) crashes, head-on crashes, crashes with trees in hazardous locations, and curve-related crashes. This guide focuses on the crash types prevalent on horizontal curves and provides objectives and strategies to improve safety on curves.

The two main objectives for improving safety along horizontal curves are to

1. Reduce the likelihood of a vehicle leaving its lane and either crossing the roadway centerline or leaving the roadway and
2. Minimize the adverse consequences of leaving the roadway.

Many of the strategies identified to achieve these objectives are common strategies to reduce ROR and head-on crashes. In cases where the guides dealing with the ROR and head-on crash emphasis areas provide thorough coverage of a particular strategy, the reader is directed to the specific sections of those guides for more detailed information. If particular issues that pertain specifically to horizontal curves are not covered in the ROR or head-on guides, these issues are discussed within the text of this guide.

The most prevalent types of crashes that occur on horizontal curves are ROR and head-on crashes; therefore, the emphasis is to reduce the frequency and severity of these types of crashes. These strategies may not eliminate crashes with other vehicles, pedestrians, bicyclists, and trains that may be directly in the path of the vehicle, but crash statistics do not indicate that these types of collisions are prevalent on curves.

Within the guide, no distinction is made as to whether a strategy is more applicable at an isolated horizontal curve located between two long tangents or whether the strategy should be applied to horizontal curves located along curvilinear alignments. In general, all of the strategies have the potential to be effective in both instances. Similarly, all of the strategies may be used in combinations to improve safety. For example, if the horizontal alignment is modified to increase the radius of a curve, it may also be appropriate to enhance the delineation along the curve.

Management of safety on horizontal curves is a major challenge for highway agencies. It has been estimated that there are more than 10 million horizontal curves in the United States on two-lane highways alone. State highway agencies generally operate accident record systems, and, within the accident record systems, accident locations can be tied to specific locations on the roadway system. Safety problems on horizontal curves can be detected using such record systems, but only indirectly because very few highway agencies have inventory files that identify the locations or geometrics of horizontal curves in a form that can be linked to accident data. Thus, safety concerns can be identified that, upon investigation, turn out to involve horizontal curves. However, there are typically no formal means of reviewing all horizontal curves and identifying those with adverse safety performance. Many local agencies do not have accident record systems in which accidents can be linked to specific roadway locations; therefore, identification of horizontal curves with potential safety concerns can only be conducted manually, if at all. Given these constraints, safety management of horizontal curves should be conducted making full use of available accident record systems. In addition, agencies that cannot identify potential problems on horizontal curves by automated means should consider the use of other methods, including noting public complaints, skid marks, and damage to roadside hardware, trees, and utility poles. Where safety concerns related to horizontal curves are found, this guide provides a range of safety improvements that can be considered.