The purpose of this document is to help Federal, State, Tribal, and local agencies understand how to incorporate the Interactive Highway Safety Design Model (IHSDM) into the road safety audit (RSA) process. This effort was comprised of three RSAs in different regions of the country, as shown in Table 1, which incorporated utilizing IHSDM in the standard eight-step RSA process. These sites were selected based on the following criteria:

1. A demonstrated need for an RSA
2. Availability of design data
3. Variation in geographic location

A more detailed report of these three RSAs is included in Appendix A.

Table 1: RSAs Conducted Using IHSDM

The primary application of IHSDM is as a decision-support tool that can be used to evaluate the safety and operation of a highway by providing a quantitative assessment of the safety implications of various design features. The results of IHSDM support decision making in the highway design process.

IHSDM consists of six modules, each of which evaluates safety from a different perspective. The following three modules were used for all three RSAs conducted for this project:

1. Policy Review Module (PRM) – Checks the geometric design elements against selected design
   policy documents.
2. Crash Prediction Module (CPM) – Estimates the frequency and severity of crashes on a roadway while taking into consideration roadway geometry and traffic characteristics with the optional use of crash history data, if available.
3. Design Consistency Module (DCM) – One design goal is to provide design consistency in order to meet driver’s expectations. This module evaluates operating speed consistency.

Although IHSDM was primarily developed as a tool to be used during the highway design process, IHSDM can be a useful tool in assisting RSA teams to make a more thorough assessment of safety. The use of IHSDM during the RSA process allowed team members to select sites for detailed review and to help prioritize areas where safety measures should be considered. Additional features that were incorporated into the RSAs by using IHSDM include the following:

• Enable the RSA team to identify potential crash factors, or combinations of crash factors, that may not be as visible or apparent to the RSA team.
• Calculate the effects of potential countermeasures in terms of expected crashes, predicted operating speeds, etc.
• Maximize the potential for the RSA team to identify and prioritize those locations that pose the greatest crash risk.

In addition to providing these quantitative assessment features, when used effectively, IHSDM is a tool that will save the RSA team time and yield a heightened awareness of safety issues. In general, the use of IHSDM helps RSA teams focus efforts, especially on longer corridors typically not suited for an RSA. However, there can be challenges to using IHSDM in the RSA process. One of the biggest challenges is obtaining and entering geometric data on post-construction (existing facility) RSAs, which can significantly increase the cost of the RSA. Considering this factor, the greatest opportunity to use IHSDM in the RSA process is where geometric data is available in electronic format or on long sections of road that would benefit from a systemic review of safety.

As the case studies in this document demonstrate, IHSDM is a useful tool in the RSA process and it is recommended that IHSDM continue to be incorporated in RSAs. Enhancements to IHSDM will facilitate this process, such as providing users with the ability to select linear referencing units (i.e., project stationing or milepost referencing) and convert between selected systems. Other enhancements include providing the ability to review IHSDM results for different modules in one graph, map, or other form of output. It is recommended that additional RSAs be conducted and documented, incorporating these enhancements and using other IHSDM modules, such as the Intersection Review and Traffic Analysis modules, not utilized as a part of the RSAs conducted for this project.