Figure 1: Idaho 8: Corridor Study Area Map

Idaho 8 Corridor

Highway Safety Manual Case Study 1: 

Using Predictive Methods for a Corridor 

Study in Idaho

HSM Reference

The Interactive Highway Safety Design Model (IHSDM) is a suite of software analysis tools 

developed by Federal Highway Administration’s (FHWA) Office of Safety Research and 

Development to evaluate safety and operational effects of geometric design decisions on 

highways. The IHSDM software includes a Crash Prediction Module that implements the 

Highway Safety Manual (HSM) Part C predictive methods for evaluating rural 2-lane highways, 

rural multilane highways and urban/suburban arterials. IHDSM may be downloaded free-ofcharge on-line at www.ihsdm.org. 

Description

The Idaho Transportation Department (ITD) 

contracted with a consultant to conduct a 

comprehensive review of existing Idaho State 

Highway 8 (Idaho 8) corridor conditions. The 

review identified deficiencies related to traffic 

operations, roadway geometry and cross section,

access control, and safety with an overall purpose to 

identify and prioritize operational improvements 

over a 10-year period. FHWA’s IHSDM was used 

to evaluate existing traffic, roadway geometry, and 

predict crashes using these and the corridor’s recent 

crash history. 

Discussion

The Idaho 8 Corridor Study evaluated approximately 11 miles of Idaho 8, from Moscow to 

Troy, Idaho. The corridor consists of a two-lane highway, characterized by rolling terrain 

traversing through primarily rural residential and agricultural development. Idaho 8 is 

classified as a minor arterial with current annual average daily traffic (AADT) ranging from 

8,600 vehicles per day (vpd) at the west end of the corridor (Moscow); to 2,600 vpd at the east 

end (near Troy). The corridor has an average of five percent heavy vehicles.
HSM Part C Case Study: IHSDM 

2

Data used to build the IHSDM model provided by ITD included “as-built” plans, five years of

historic crash data, video logs, traffic control, and existing and projected traffic volumes. These 

datasets were translated into a tabular format for incorporation into the Crash Prediction 

Module.

Figure 2: CPM Model Output: Crash Prediction Summary

The Crash Prediction Module (CPM) of the IHSDM implements the Highway Safety Manual 

predictive methods included in Part C for rural two-lane highways, rural multilane highways 

and urban/suburban arterials. Figure 2, above, is an example of the CPM for the Idaho 8 

Corridor Study. The HSM Part D crash modification factors (CMFs) were used to compare 

effects of alternative mitigation strategies. A traffic operations review using the procedures 

outlined in the Transportation Research Board’s Highway Capacity Manual for two-lane 

highways supplemented this analysis.
HSM Part C Case Study: IHSDM 

3

Expected Crash Rates - Section 1 

Table 1: Idaho 8 CPM Model Output

The CPM required coding for each segment of highway as defined by horizontal and vertical 

geometry, lane and shoulder width, roadside hazard rating, driveway density, intersection type

and location. Crashes were geo-referenced with the roadway geometry. For the 11 mile Idaho 8 

project, the time to code the existing conditions and quality-check coding was approximately 

100 hours of effort. The users encountered gaps in the availability of existing data. In most 

cases, the IHSDM accommodated these data gaps by providing default values. Quality control 

in data entry was critical. Users were alerted to inconsistencies when running the modules 

through issue summaries. Users were alerted to

noticeable anomalies in geometric elements when 

referencing the map view in the program.

The CPM model output (seen in Table 1) provides

graphs and tables containing the expected number 

of crashes and frequencies for every highway 

element and at aggregated level. From the output 

Idaho officials found that more than half of the 11-

mile corridor exhibited a calculated crash rate higher 

than the statewide average. The expected crash 

locations are a reflection of historic crash data and 

existing physical conditions within the corridor. 

This information was used to help isolate potential 

improvement locations. Once problematic locations 

were determined, contributing conditions were 

further examined to arrive at probable mitigation 

measures. The list of mitigation strategies were 

evaluated and prioritized based upon a variety of 

criteria including the likelihood of future crash

reduction at these locations.

Training Needs

The project team received training to become familiar with the use and application of each of 

the six modules of the IHSDM. In addition to the CPM, this training included the Policy 

Review Module (PRM), Design Consistency Module (DCM), Traffic Analysis Module (TAM),

Intersection Review Module (IRM) and Driver-Vehicle Module (DVM). Training sessions 

included detailed discussions about understanding the capabilities of each module, data input 

requirements, and interpretation of results. The ITD District Two staff provided data input and 

supported the IHSDM application on the corridor study.

Benefits

The Idaho 8 Corridor Study was completed in 2010 within a 10-month time span. While the 

IHSDM is a data intensive program, the time and effort invested in data entry is rewarded 

through the production of a vast amount of detailed data output from the modules. Data 

required to run the IHSDM CPM module are the same data required to run HSM predictive 

methods. All IHSDM modules, except the IRM, were run as part of the alternative analysis. Of 

First Year of Analysis 2016

Last Year of Analysis 2020

Evaluated Length (mi) 11.39

Average Future Road AADT (vpd) 4,114

Total Crashes 153

Fatal and Injury Crashes 56

Fatal and Serious Injury Crashes 36

Property-Damage-Only Crashes 96

Percent Fatal and Injury Crashes 37%

Percent Fatal and Serious Injury Crashes 24%

Percent Property-Damage-Only Crashes 63%

Crash Rate (crashes/mi/yr) 2.68

Fatal and Injury Crash Rate (crashes/mi/yr) 0.99

Fatal and Serious Injury Crash Rate (crashes/mi/yr) 0.64

Property-Damage-Only Crash Rate (crashes/mi/yr) 1.69

Total Travel (million veh-mi) 85.5

Travel Crash Rate 1.78

Travel Fatal and Injury Crash Rate 0.66

Travel Fatal and Serious Injury Crash Rate 0.42

Travel Property-Damage-Only Crash Rate 1.13

Expected Crashes 

Expected Crash Rate 

Percent of Total Expected Crashes 

Expected Travel Crash Rate (crashes/million veh-mi)
HSM Part C Case Study: IHSDM 

4

these five modules, the IHSDM PRM and CPM modules were the most useful in identifying

existing geometric deficiencies, specific locations needing further evaluation, or locations 

needing possible design improvements, and potential safety issues. IHSDM software produced

a variation of graphical outputs for each review, which were modified to meet specific data or 

formatting needs. 

Recommended mitigation measures consisted of access management strategies, development of 

passing lanes, increased intersection capacity, sight distance improvements, roadside safety 

enhancements, intelligent transportation system (ITS) applications, and development of animal 

crossings. A Corridor Plan Report was prepared to document the review, analysis, and 

resulting recommendations of this study to be included in the Statewide Transportation 

Improvement Program (STIP) for future implementation.

Contact

Ken Helm

Senior Transportation Planner

Idaho Transportation Department

(208) 799-4229

ken.helm@itd.idaho.gov

“The advantage of employing IHSDM on this 

project was the opportunity to perform a 

detailed and simultaneous review within the 

corridor on a variety of critical elements (that 

is, traffic operations, geometry, safety) to 

isolate potential problem areas and allow 

development of strategic mitigation 

strategies.” – Bob Beckman, Project Manager