There were three IHSDM modules that were run prior to each RSA. The results of the analysis were used to identify locations of interest for each RSA team. IHSDM output included the following:

1. Policy Review Module: Stopping Sight Distance, Passing Sight Distance, Length of Curve, and Radius of Curve – Other PRM checks were not run since the exact design data was not available or the effects of roadway geometry (e.g. roadway widening) was outside of the scope of the project due to right-of-way constraints.
2. Design Consistency Module: Understanding the difference between estimated 85th percentile speeds vs. the design speed and the expected reduction in estimated 85th percentile speeds from an approach tangent to succeeding horizontal curves were seen as critical given the crash history and alignment characteristics.
3. Crash Prediction Module: The CPM was run using historical crash data (Empirical Bayes Analysis), which is recommended for locations that have similar characteristics in the "before" and "after" analysis periods. According to the HSM: "If the analysis is being performed to assess the expected average crash frequency of a specific highway facility, but is not part of the analysis of a planned future project, then the EB Method should be applied." The Empirical Bayes (EB) procedure combines expected crash frequencies (estimated using the SPFs and CMFs) with site-specific crash history data. The advantage of using the EB procedure is to improve the accuracy of estimates for an individual location by factoring in the actual crash history of the location being evaluated. In theory, when enough years of crash history data are available, it would be more accurate to appropriately combine estimates from the SPFs and CMFs with site-specific crash history data than to rely on either the model estimates or the site-specific data alone. The EB procedure determines the statistically appropriate weighting of estimates from the SPFs and CMFs and site-specific crash history data.

The IHSDM analysis produced output tables and graphs that illustrate locations where stopping sight distance was not met, policy values for horizontal curve length and radius were not met, the 85th percentile speed profile for the corridor traveling in each direction, any accompanying "flagged" elements, and projected crashes along the alignment.

For the purposes of comparison, IHSDM results were annotated on an aerial map to compare the results across different modules. This was done to help the RSA team focus on the most critical issues. While there is no set manner of determining which factors (expected crash rate, speed differential, etc.) should be weighted more during the RSA, and all of the locations that rank highly in the IHSDM analysis may not coincide with historical crash locations, the IHSDM results provide information on which segments warrant closer inspection. Also, where these locations show similar attributes to other high-incidence locations on the corridor, similar safety treatments may help to preemptively address future crashes.

Benefits

Using IHSDM during the RSA process resulted in several benefits. These are described in this section.

The use of IHSDM helps RSA teams focus efforts, especially on longer corridors typically not suited for an RSA. An RSA is conducted by a multidisciplinary team who evaluates factors that may affect the safety performance evaluation of an existing or future roadway. The RSA team consists of skilled professionals in a field that influences roadway safety such as engineers, law enforcement, maintenance, and other professionals. As such, the time an RSA team can devote to a particular project or event is likely limited. IHSDM provides a quantitative measure of safety, which, along with crash and other data, helps focus a RSA team's efforts on critical conditions or areas. As a result the RSA team can more effectively use their time to evaluate longer corridors or corridors where little crash data are available.

IHSDM offers a quantitative methodology to assess safety. The opportunity to have quantitative assessment of safety along with a multidisciplinary RSA team provides an excellent opportunity for not only developing potential engineering strategies to improve safety, but also provides an opportunity to review various programs, policies, and practices and their effect on safety. This can be a critical aspect of addressing safety as effective utilization and engagement of the four E's of safety – engineering, enforcement, education, and emergency services – is the basis for improvement.

Among the three modules used for the three RSAs for this project, the CPM was found to have the greatest impact on the RSA team on the RSAs conducted in Rhode Island and Montana. This confidence in the CPM may be because it faithfully implements HSM Part C (Predictive Method). Specifically, the CPM provides the following benefits:

• 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 reduced crashes.
• Maximize the potential for the RSA team to identify and prioritize those locations that pose the greatest crash risk.

The PRM and DCM results were also considered by the RSA team. The DCM helped identify locations that may violate driver expectancy, and the PRM stopping sight distance policy check identified locations where sight distance may be limited or critical given other conditions observed in the field. The other PRM policy checks did not have as significant of an impact, likely because two RSAs were on existing roads where a redesign of the roadway (flattening horizontal curves) was outside the RSA scope and on the third RSA, which was a roadway improvement project, a redesign (also flattening horizontal curves) would have significant cost implications because of the mountainous terrain.

IHSDM supports a systemic approach to addressing safety. On each of the three RSAs studied as part of this project IHSDM output "flagged" horizontal curves and intersections that were subsequently reviewed by the RSA team in detail. This method resulted in recommended strategies that were appropriate for the entire corridor, not just at specific locations where crashes had occurred. Had IHSDM not been utilized, the tendency of the RSA team may have been to consider strategies at the high crash locations only. In other words, using IHSDM helped promote a systemic approach to addressing safety.

Challenges

There are multiple benefits of using IHSDM during the RSA process. However, there are also factors that make using it during the RSA process more challenging. These challenges are described in
this section.

Data entry can be intensive if electronic design files are not available. Roadway geometry data can be imported into IHSDM from most design software by exporting to LandXML from the design software and then importing the LandXML data into IHSDM. While pre-construction RSA geometry data are available, post-construction RSAs (existing facilities) may not have the geometric data that is required to fully utilize IHSDM readily available. Data requirements vary by module but all require basic alignment and cross sectional data. In cases such as these, data can be obtained from as-built drawings and entered manually or obtained by using aerial photos, GIS Data, or elevation information to create a "best fit" design of the alignment using design software so that the geometric data can be imported into IHSDM. The drawbacks of using this method include the time required to create a surrogate design. A surrogate design may not precisely describe existing geometric conditions, which may introduce error into the IHSDM output. Furthermore, some geometric information cannot be obtained without extensive field data collection, requiring estimates for the values. The most notable of these data are superelevation, roadside conditions, and obstruction offsets. All three RSAs conducted as part of this project required these values to be estimated.

It should be noted that the sensitivity of some of the estimated inputs do not have a dramatic effect on IHSDM output and that conditions estimated for the entire length of a roadway would not affect the relative output values from IHSDM. In other words, despite the possible inaccuracy of geometric data estimated from multiple sources, this did not have a significant impact on the results overall. The effort to gather or estimate required data will increase the cost of the RSA, which may make the RSA cost prohibitive. For this reason, 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.

Roadway geometry data must be in project stationing format in order to use IHSDM. Since all three RSAs were conducted on existing facilities (along with a proposed redesign of the roadway in the case of the RSA conducted in Oregon), all roadway geometric data were referenced by milepost to relate to conditions in the field. This required converting all geometric data between project stationing and milepost referencing to relate IHSDM output to conditions in the field, thus, increasing the effort to utilize IHSDM. The complexity of this conversion process was increased when station equations – locations where alignment stationing is adjusted, resulting in locations with separate forward and backward direction stationing – were part of the geometric attributes of the roadway.

Two of the three RSAs conducted as part of this project had a significant number of station equations along the alignment. Only the Rhode Island RSA did not have station equations because the RSA team set up user-defined "dummy" stationing to analyze the alignment. The project stationing for this analysis was continuous without breaks (station equations), which facilitated interpretation of results and comparison to conditions in the field.

Opportunities

In addition to providing the ability for the user to define the geometry referencing system used (e.g., stations, milepost, etc.), the ability to compare IHSDM results across modules was seen as an opportunity for enhancement. IHSDM provides module-specific outputs in the form of tables and graphs. For two of the RSAs conducted as part of this pilot (Rhode Island and Montana), IHSDM output for all modules utilized were annotated on aerial maps and displayed in tables to enable RSA teams to compare results across modules and identify priority site visit locations. Similar side-by-side comparisons of IHSDM output could be made with graphical output as well.