SAFER ROADS THROUGH BETTER DESIGN

Improving roadway safety continues to be at the forefront of all stages 

of project development, including planning, alternatives analysis, design, 

construction, and operations. Advanced analysis methods and tools 

have been developed to help practitioners improve safety. One tool 

available now to estimate a project’s substantive safety is the Interactive 

Highway Safety Design Model (IHSDM). IHSDM supports performancebased practical design (PBPD) via implementation of Highway Safety 

Manual (HSM) Part C predictive methods and can be an integral part of 

the Road Safety Audit process.

The IHSDM contains six modules: Crash Prediction, Design Consistency, 

Intersection Review, Policy Review, Traffic Analysis, and Driver/Vehicle. 

This FREE software package helps users identify areas of safety concern 

within a given facility, and IHSDM outputs assist agencies in determining 

how to best invest limited resources to improve safety performance. 

Results from the model help project developers identify and justify safetyrelated design decisions. 

Who should use the IHSDM?

` Project analysts, designers, 

and managers

` State and local highway 

agencies and their 

consultants

` Transportation research and 

academic communities

Using the Interactive Highway Safety Design Model

[ 1 ]
FHWA-SA-17-007

Why use the IHSDM CPM to implement 

HSM Part C methods?

 Applies to all facility types covered in HSM 

Part C. 

 Offers the ability to import data from CAD 

files rather than enter in data by-hand.

 Automatically segments highways for 

evaluation using HSM Part C segmentation 

rules.

 Uses the Empirical-Bayes process.

 Implements the calibration procedures 

described in the Appendix to HSM Part C.

 Allows agencies to enter their own Safety 

Performance Functions.

 Graphically displays crash and roadway 

data outputs, allowing users to quickly and 

easily identify potential safety concerns. 

Using the Crash Prediction Module to Implement HSM Part C Methods

Since 2010, practitioners have used the 

HSM to estimate predicted crash frequency 

and severity based on the characteristics 

of a particular roadway.1 The IHSDM’s crash 

prediction module (CPM) – which incorporates 

the most current analytical methods – directly 

implements Part C of the HSM to:

` Predict crash frequency for highway 

segments, intersections, and interchanges;

` Evaluate the safety effects of highway 

improvements and treatments;

` Compare relative safety performance of 

design alternatives; and

` Assess the safety and cost-effectiveness of 

design decisions.2

The CPM has the ability to evaluate all roadway types covered in HSM Part C, including rural two-lane 

highways, rural multilane highways, urban/suburban arterials, freeway segments, and freeway ramps/

interchanges (including ramps, collector-distributor roads and ramp terminals). It estimates the frequency and 

severity of expected crashes at a location based on its geometric design and traffic characteristics.3

Supporting Performance-Based Practical Design

In recent years, agencies have established increasingly 

flexible project design criteria. The Federal Highway 

Administration (FHWA) has encouraged PBPD in an effort 

to ground cost-saving design decisions in a performancemanagement framework.4

 Several agencies have used 

the IHSDM to evaluate and optimize their design decisions 

as they address a project’s purpose and need. The 

IHSDM helps engineers balance short- and long-term 

project goals while considering operational and safety 

performance.

For example, a designer determines that the elimination of 

shoulders and a reduction in lane widths from 12 ft. to 10 

ft. will add space for a needed turn lane without the need 

to acquire right-of-way. Using the IHSDM Crash Prediction 

Module (CPM), the designer can weigh the overall safety 

impacts of reducing lane and shoulder widths (which may 

increase lane departure crashes) and adding a turn lane 

(which may reduce crashes for turning vehicles). 

Predicted Crashes from 2018 to 2038

Total for I-70

Huntington St

Markeys Rd

SR 3014

SR 3037

53.0

2.5 16.5

2.0

2.0

3.0

41.0

6.0

3.5

3.5

3.5 1.5 1.5

19.5

5.5

5.5

4.5

9.0

46.5

17.5

SR 3010

70

423

CRASHES

Predicted crash frequency by segment for an 

example highway project alternative.

[ 2 ]

Predicted Crashes from 2018 to 2038

SAFER ROADS THROUGH BETTER DESIGN

Using the Interactive Highway Safety Design Model
Nevada SR 147: 

Evaluating Safety Performance Using the IHSDM

The Nevada DOT (NDOT) used the IHSDM CPM to apply the HSM predictive method on a rural two-lane section 

of Nevada State Route (SR) 147 for a 20-year period (2013-2033). The agency targeted this 7-mile corridor 

because over an 8-year period, there were 39 crashes, including 7 fatal crashes. A high number of crashes 

occurred at a single reverse curve at milepost (MP) 9. The IHSDM CPM allowed the agency to evaluate and 

prioritize safety improvement alternatives by estimating expected crash totals for each alternative. NDOT 

evaluated six safety improvement project alternatives:

1. Add centerline rumble strips for the 

entire length of project.

2. Improve horizontal geometry for a single 

curve at MP 9, including shoulder and 

roadside improvements.

3. Improve superelevation to bring 

it into compliance with AASHTO 

recommendations.

4. Add a climbing lane in one direction for 

a 3.1 mile section, including shoulder 

and roadside improvements.

5. Redesign the entire reverse curve at 

MP 9, including shoulder and roadside 

improvements.

6. Widen shoulders from 1 ft. to 5 ft. and improve roadside conditions for the entire length of project.

Analysts entered data for the existing condition and for each alternative into the IHSDM CPM to predict the 

crashes for the next 20 years based on the geometry, projected traffic volumes, and crash history. Projected 

reductions in the number of total crashes for the study period ranged from 1 crash for Alternative 3 to 19 

crashes for Alternative 6. 

NDOT compared the reductions in expected crashes to the treatments’ cost and performed an economic analysis, 

including benefit-cost (B/C) ratios, for each alternative. Analysts calculated the monetary value of the crash 

reduction based on the Nevada 2012 societal cost for crashes. Alternative 1 exhibited the highest B/C ratio (28.4). 

Alt. 1 Alt. 2 Alt. 3 Alt. 4 Alt. 5 Alt. 6

Reduction in Total Crashes 5.4 4.9 1.0 14.2 5.8 19.2

Cost $66,000 $1,475,000 $945,000 $5,696,000 $2,358,000 $12,114,000

B/C Ratio 28.4 2.6 0.8 1.8 1.8 1.2

This project was presented at the TRB 2013 Annual Meeting. A poster of the project is available for download here: http://www.ihsdm.

org/w/images/c/c5/022013_TRB_Poster_NDOT_Final_Mosley.pdf. 

SAFER ROADS THROUGH BETTER DESIGN

Using the Interactive Highway Safety Design Model

Graphical Representation of Alternates 2, 4, and 5 for 

Reverse Curve at MP 9.

[ 3 ]
How does the IHSDM CPM differ from traditional spreadsheet tools? 

Compared to other tools such as the Interchange 

Safety Analysis Tool – Enhanced (ISATe), the IHSDM CPM 

provides significant benefits by: 

` Automatically segmenting highways for evaluation 

through “station-based” data input and analysis,

` Offering more robust reporting capabilities than 

traditional tools,

` Analyzing both freeway and non-freeway network 

components (e.g., cross-roads, connectors, local 

roads, and intersections), and

` Graphically displaying which design features will yield 

the greatest safety benefit.

The IHSDM CPM also allows the user to import project alignment data from highway design software via 

LandXML. Data can be input and analyzed using two methods:

Station-based: automatically segments the highway into homogeneous segments, saving time and reducing 

the likelihood of user error.

Site-based: functions similar to a spreadsheet tool and is especially useful for projects where detailed, 

station-based geometry is not available. 

Expected Crashes for Curves at MP 9

Mile Post

8.50

8.60

8.70

8.80

8.90

9.00

9.10

9.20

9.30

9

8

7

6

5

4

3

2

1

0

Expected Crashes by Horizontal Design Element

Curve 2

Tangent Curve 3 Curve 4 Curve 5

Short Tangent Between 

Compound Curves

Tangent

Tangent

New Curve 4 - Alt. 2

New Curve 4 - Alt. 5

New Curve 3 - Alt. 5

Exist

Alt. 1 

Alt. 2 

Alt. 3 

Alt. 4 

Alt. 5 

Alt. 6 

Output derived from IHSDM results graphically displays changes in the expected number of crashes for each alternative.

Nevada SR 147 Expected Crashes for Curves near MP 9

Automates the 

evaluation process 

for the user

Allows agencies 

to make more 

informed design 

decisions

Uses current 

analytical 

methods, 

oering robust 

reporting

LESS TIME

& MONEY

BETTER 

TARGETED 

INVESTMENTS

FEWER 

FATALITIES 

& SERIOUS 

INJURIES

[ 4 ]

SAFER ROADS THROUGH BETTER DESIGN

Using the Interactive Highway Safety Design Model
Using the IHSDM to Evaluate Alternative Interchange Designs 

The Pennsylvania Department of Transportation (PennDOT) submitted a Conceptual Point of Access (CPOA) 

request to the FHWA Pennsylvania Division Office for a section of I-70 in Western Pennsylvania, which 

encompasses two closely spaced interchanges. The CPOA eventually focused on two primary alternatives:

` Alternative 2A retained 

and redesigned the two 

closely spaced interchanges 

with some I-70 mainline 

improvements.

` Alternative 3 removed one of 

the interchanges, redesigned 

the other, and included a 

new connector road and 

local roadway network 

improvements.

While both alternatives appeared 

to be acceptable based on 

engineering and operations 

review, the FHWA Interstate 

System Access Informational 

Guide requires “that the proposed 

change in access does not have 

a significant adverse impact on 

the safety and operation of the 

Interstate facility.”

The FHWA Geometric Design 

Laboratory (GDL) compiled 

the necessary data – including 

average annual daily traffic 

(AADT), crash data, and 

alternative geometric elements – 

to evaluate the alternative designs 

for a 20-year period (2018-2038). 

Analysts used the IHSDM CPM 

to evaluate each entity (freeway segments, ramps, ramp terminals, crossroads, and intersections) for both 

alternatives. Alternative 2A was superior to Alternative 3 in expected safety performance, in that:

` Expected crashes over 20 years for Alternative 2A were approximately 10 percent fewer than for Alternative 3. 

` An analysis of fatal and injury (crashes) showed that Alternative 2A would experience an estimated 

15 percent fewer of these crashes than Alternative 3.

This project was presented at the TRB 2016 Annual Meeting. A poster of the project is available for download here: http://ihsdm.org/w/

images/c/c8/P16-1759_PA_Use_of_IHSDM_for_I-70_Safety_Analysis.pdf.

Huntington St SR 3014

SR 3037

SR 3010

70

Plan view of Alternative 2A in IHSDM Highway Viewer

Markeys Rd

Connector Rd

Huntington St SR 3014

SR 3037

SR 3010

70

Markeys Rd Plan view of Alternative 3 in IHSDM Highway Viewer

[ 5 ]

SAFER ROADS THROUGH BETTER DESIGN

Using the Interactive Highway Safety Design Model
Using the IHSDM in Road Safety Audits 

In addition to agencies using the IHSDM to improve 

highway design safety, practitioners have found that the 

IHSDM is a useful tool during the Road Safety Audit (RSA) 

process. When RSA teams use the IHSDM modules prior 

to their assessment, the outputs can help practitioners 

flag potential problems for detailed field investigation 

and point them to measures that may mitigate safety 

issues identified during the RSA. The IHSDM can also help 

focus an RSA team’s efforts, maximizing the efficiency 

and productivity of the audit process. 

In a recent series of RSAs conducted in Montana, 

Oregon, and Rhode Island, IHSDM results enabled the 

RSA teams to identify potential crash factors in advance 

of the RSA and to calculate the effects of potential 

countermeasures in terms of expected crashes, 

predicted operating speeds, stopping sight distance, 

etc. It also improved the potential for the RSA team 

to identify and prioritize those locations that pose the 

greatest risk of future crashes, not just locations with a 

history of reported crashes.5

 Pairing the IHSDM with RSAs

FHWA and the Oregon Department of Transportation (ODOT) partnered to conduct an RSA along US 

Highway 97 in south central Oregon, between Modoc Point and Algoma in Klamath County. ODOT 

noted the presence of narrow shoulders, steep cut and fill slopes, and fixed objects within the clear zone, 

including barriers, guardrail, and guardrail end terminals that did not meet current standards.

The RSA team reviewed safety conditions on US 97 using recent crash data, local and RSA team inputs, 

and the IHSDM. The IHSDM enabled the RSA team to graphically compare the existing available stopping 

sight distance and the stopping sight distance associated with the proposed design. It also predicted 

crashes along the future roadway based on its geometric characteristics and projected traffic volumes. 

The IHSDM highlighted locations where drivers may need to decelerate to safely negotiate a horizontal 

curve, thereby assisting in the prioritization of locations for RSA team review.

The ability to quantify values associated with design issues (e.g., stopping sight distance) and to estimate 

future crash frequencies based on the road redesign enhanced the value of the RSA by demonstrating 

the magnitude of the safety issues identified by the RSA team. Without the IHSDM estimates, the RSA 

team could identify–but could not necessarily quantify–expected changes in safety parameters such as 

operating speeds or expected crash frequencies. Quantifying these parameters and frequencies provided 

all parties (the RSA team, Design Team, and Owner) a higher level of confidence in the magnitude and 

importance of the safety issues identified in the RSA.

The RSA team identified several benefits 

of using the IHSDM in the RSA process. 

The model:

 Helps RSA teams focus their efforts, especially 

on longer corridors where completion of an 

RSA would be time and cost prohibitive.

 Offers a quantitative methodology to assess 

safety that:

•Enables the RSA team to identify potential 

crash factors, or combinations of crash 

factors, that may not be readily visible or 

apparent.

• Calculates the effects of potential 

countermeasures in terms of reduced 

crashes.

•Maximizes the potential for the RSA team 

to identify and prioritize those locations 

that pose the greatest crash risk.

[ 6 ]

SAFER ROADS THROUGH BETTER DESIGN

Using the Interactive Highway Safety Design Model
Prole (ft)

Vertical Curves

Horizontal Curve

Radius (ft)

Degree of Curve (deg)

Crashes by 

Homogenous 

Segment 

(crashes/mi/yr)

Crashes by 

Horizontal Design 

Element

(crashes/mi/yr)

Design Speed (mph)

V85 Speed (mph)

Posted Speed (mph)

Station

50+00 100+00 150+00 200+00 250+00 300+00 350+00 400+00

Design Consistency Module Crash Prediction Module

Policy 

Review Module Highway Data (Input)

Intersection with Chopmist Hill Road (Horizontal Curve)

Required Sight 

Distance (ft)

Available Sight 

Distance (ft)

This IHSDM output visually flags potential problem locations for further investigation by practitioners. The output 

corresponds to a RSA performed on US Highway 2 in Kalispell, Montana.

RSA IHSDM Output

[ 7 ]

SAFER ROADS THROUGH BETTER DESIGN

Using the Interactive Highway Safety Design Model
Every Day Counts (EDC), a State-based initiative of FHWA’s Center for Accelerating Innovation, 

works with State, local and private sector partners to encourage the adoption of proven 

technologies and innovations aimed at shortening and enhancing project delivery.

www.fhwa.dot.gov/everydaycounts FHWA -SA-17-011

Additional Resources and Available Training

The most recent version of the IHSDM – HSM Predictive Method Release can be downloaded for free at: 

http://www.ihsdm.org after completing a brief online registration and providing e-mail contact information. 

Federal Highway Administration’s Geometric Design Laboratory (GDL) at the Turner-Fairbank Highway Research 

Center provides free technical support for IHSDM users.

Website: https://www.fhwa.dot.gov/research/tfhrc/labs/geometric/

Contact: IHSDM.Support@dot.gov and (202)493–3407

Free training on HSM Part C as well as tools such as the IHSDM. 

Website: http://safety.fhwa.dot.gov/rsdp/training.aspx

Contact: Gene Amparano

Road Safety Audit Case Studies: Using IHSDM in the RSA Process shows full results of three RSA case studies and 

is located at: http://safety.fhwa.dot.gov/rsa/case_studies/fhwasa14071/. 

More resources are available in the IHSDM Library at: https://www.fhwa.dot.gov/research/tfhrc/projects/safety/

comprehensive/ihsdm/libweb.cfm. 

Abdul Zineddin 

Manager, Geometric Design Lab (GDL)

Office of Safety R&D, FHWA 

Abdul.Zineddin@dot.gov 

Gene Amparano 

Resource Center, FHWA 

Gene.Amparano@dot.gov

Jerry Roche 

Office of Safety, FHWA 

Jerry.Roche@dot.gov 

IHSDM KEY CONTACTS

1 American Association of State Highway and Transportation Officials, “An Introduction to the Highway Safety Manual,” n.d. Available at: http://www.

highwaysafetymanual.org/Documents/HSMP-1.pdf..

2 Federal Highway Administration, Office of Research and Technology, “Interactive Highway Safety Design Model (IHSDM): Modules” web page. Available 

at: https://www.fhwa.dot.gov/research/tfhrc/projects/safety/comprehensive/ihsdm/modules.cfm.

3 Federal Highway Administration, Office of Research and Technology, “Interactive Highway Safety Design Model (IHSDM): Crash Prediction Module” web 

page. Available at: https://www.fhwa.dot.gov/research/tfhrc/projects/safety/comprehensive/ihsdm/crashweb.cfm. 

4 Federal Highway Administration, Office of Design,“Performance-Based Practical Design – Frequently Asked Questions” web page. Available at: 

https://www.fhwa.dot.gov/design/pbpd/general_information/faqs.cfm. 

5 Adapted from Rebecca Crowe, “Interactive Highway Safety Design Model Helps RSA Teams Identify Issues,” Safety Compass, Volume 9 Issue 1. Available 

at http://safety.fhwa.dot.gov/newsletter/safetycompass/2015/winter/#s5.

SAFER ROADS THROUGH BETTER DESIGN

Using the Interactive Highway Safety Design Model