Design standards are applied by highway agencies to ensure optimal operational and safety performance based on the anticipated use of roads in their system. Ideally, the application of the highest design standards would be expected to maximize safety. However, when operating under constraints, compromises on standards become necessary. A better understanding of the incremental and combined effects of roadway design features on safety is needed to guide decision making and promote highway safety management efforts. A considerable amount of research has been conducted to understand the effects of geometric and traffic features on safety. The results of that research need to be synthesized into a unified document and correlated with current design practice to allow development of a hierarchy of the relative safety benefits of highway design features. This will enable highway agencies to select design features that are essential to highway safety and allow comparisons among alternative investment policies that will optimize the overall safety of their highway systems in the real world of limited resources and other constraints.

The objective of this research is to assess the safety effects of highway design standards and to synthesize the findings into documents that will provide guidance in addressing safety needs, given limited resources and other constraints. The research should be limited to geometric, cross-sectional, and roadside design elements for all roadway types and environments. The composition of traffic and seasonal and daily variations is considered an important aspect of this research. The research involved (1) review of recent studies to identify the critical variables and parameters associated with the relationship among design features and highway safety, (2) contacts with road design agencies to review their processes and practices for the application of design standards for different classes of roads, varying traffic conditions, and other factors (e.g., safety analyses, design exception reports), (3) critical review of the information gathered to assess the usefulness of the relationships between safety and design features, the confidence in the data used to establish the relationships, the range of applicability, the soundness of procedures to apply the relationships, and the consensus among agencies, and (4) synthesis of the information relating the effects of particular features on highway safety under varying conditions. The synthesis effort revealed that significant gaps exist in basic knowledge making it impossible for the project to make recommendations for revisions or additions to the Greenbook. An ordered list of research plans to address the deficiencies identified was prepared.

Status: The revised final report has been published as NCHRP Report 374, "Effect of Highway Standards on Safety." Research to address some of the identified gaps in knowledge is being undertaken as NCHRP Project 17-9(2).