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Highway Safety Manual Case Study 4: 

Development of Safety Performance Functions for 

Network Screening in Illinois

HSM Reference

Safety Performance Functions (SPF) are statistical models used to estimate the average crash 

frequency for a specific site type (with specified base conditions), based on traffic volume and

roadway segment length. Part B of the American Association of State Highway and 

Transportation Officials’ (AASHTO) Highway Safety Manual (HSM) demonstrates the use of 

SPFs for network screening, while Part C uses SPFs to predict the number of crashes for a 

particular site given specific “base” conditions.

SPFs are developed through statistical regression modeling using historical crash data. 

Although they require more data, SPFs developed using the Empirical Bayes (EB) approach are 

more sophisticated than crash frequency or crash rate as a predictor because they account for 

the regression to the mean bias and random fluctuations in crash data over any period of time.

Across the country, SPFs have been developed for a variety of analysis purposes and should be

tailored to the analysis being completed and the available data to ensure the most reliable 

results. 

SPFs used for network screening are more general and require less data than the predictive 

methods and allow agencies to identify high-priority locations for potential improvement. The 

SPFs in Part C of the HSM were developed based on multistate data and require a local 

calibration factor to accurately apply them to specific locations. The HSM provides instructions 

on the calibration process and safety analysis tools such as SafetyAnalyst and the Interactive 

Highway Safety Design Model (IHSDM) automatically calibrate the models. Although models 

based on multistate data sets are helpful, they produce generalized results and may not be as 

accurate for localized conditions. Alternatively, agencies with robust data sets may develop 

state-specific SPFs to model their conditions more precisely.

Description

In 2006, the Illinois Center for Transportation, Illinois Department of Transportation (IDOT) and 

the University of Illinois at Urbana-Champaign performed a research project to develop statespecific SPFs to use in the State’s network screening process. Prior to 2006, IDOT used the High 

Accident Location Identification System (HALIS) to identify High Accident Locations (HAL) 

and wet pavement cluster/segments that were based on crash frequency, rate, and equivalent 

property only. This project addressed the following specific challenges: 1) linking crash and 

roadway data, given each had a different linear referencing system; 2) establishing the criteria to 

assign each roadway segment to a peer group; 3) using “reference points” to establish 

intersection locations; 4) currency of crash and traffic data; 5) processing a large volume of crash 

and roadway data; and 6) helping users understand the meaning of the results.
HSM PART B CASE STUDY: 

SAFETY PERFORMANCE FUNCTION DEVELOPMENT

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Discussion

The Federal Highway Administration/National Highway Institute course, New Approaches to 

Highway Safety Analysis, inspired IDOT to begin the transition to more scientific approaches to 

transportation safety analysis. The team reviewed the available data and modeled the 

development of SPFs after the SafetyAnalyst peer groups (for example, rural two-lane highways, 

rural multilane divided highways, etc.). The fundamental roadway data elements were used to 

define roadway segments or intersections into peer groups, as shown in Table 1, and crashes 

were assigned to the appropriate roadway segments or intersections.

Table 1: Illinois SPF Peer Groups

Segments Intersections

1 – Rural two-lane highways 1 – Rural minor-road STOP control

2 – Rural multilane undivided highways 2 – Rural all-way STOP control

3 – Rural multilane divided highways 3 – Rural signal control

4 – Rural freeways – four lanes 4 – Rural undetermined 

5 – Rural freeways – six-plus lanes 5 – Urban minor-road STOP control

6 – Urban two-lane arterials 6 – Urban all-way STOP control

7 – Urban multilane undivided arterials 7 – Urban signalized

8 – Urban multilane divided arterials 8 – Urban undetermined

9 – Urban one-way arterials

10 – Urban freeways – four lanes

11 – Urban freeways – six lanes

12 – Urban freeways – eight-plus lanes

SAS statistical software was used to develop negative binomial equations with over dispersion 

parameters for each of the peer groups for three crash categories: fatal crashes, A-injury or 

incapacitating injury crashes, and B-injury or non-incapacitating injury crashes, resulting in 60 

models in total. The models were based on crash frequency of state highways for the years 2001 

to 2005 with available average daily traffic (ADT).

As shown in Figure 1, the SPFs provide the predicted number of fatal, A-injury type, or B-injury 

type crashes for a given segment or intersection based on the annual ADT. The EB technique 

was applied to adjust the observed number of crashes by severity at a segment or intersection. 

The difference of the EB adjusted average crash frequency and the predicted average crash 

frequency from a SPF is referred to as the potential for safety improvement, or PSI. 

The HSM refers to this value as the excess expected average crash frequency.
HSM PART B CASE STUDY: 

SAFETY PERFORMANCE FUNCTION DEVELOPMENT

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“We produced safety performance functions based on the 

data we had available which has been applied to our entire 

state system to provide network screening results and high 

priority locations for further investigation and project 

development. The Part C Highway Safety Manual SPFs 

with the Empirical Bayes technique used for crash 

prediction requires additional roadway data, but has 

improved the sophistication of safety analyses in Illinois 

resulting in better decisions to allocate limited safety 

resources.”

- Dave Piper, Illinois Department of Transportation

IDOT put further emphasis on the most severe crashes by applying a weighting factor of 25 

times a fatal crash, 10 times a A-injury crash, and one time a B-injury crash to obtain a weighted 

PSI for each roadway segment or intersection. Intersection sites were ranked sequentially from 

highest PSI to lowest PSI 

by peer group to identify 

high-priority locations for 

further review and 

potential implementation 

of safety countermeasures. 

Sliding window analysis 

was performed on the 

roadway segments to 

aggregate the 

performance measure 

over one rural mile

(minimum) or 0.25 urban 

mile (minimum). The analysis segments were shifted incrementally to identify the locations 

with the most severe needs. Ultimately, overlapping analysis segments were removed to obtain 

one unique value for each roadway segment. The PSI values for roadway segments were also 

ranked from highest to lowest to identify the routes with the greatest potential to respond to 

safety improvements.

Training Needs

Although IDOT did not conduct formal training to cover the actual analysis, the Bureau of 

Safety Engineering met with each of the IDOT Districts to discuss the process and make sure the 

District staffs had a basic understanding of the terminology and the process.

Coordination with the District staffs also focused on using the results of the new screening 

techniques, including identifying contributing factors and developing recommendations for 

potential countermeasures to address the locations with high PSI values.

Benefits

A variety of benefits have 

emerged from the 

development of SPFs for 

Illinois. The use of SPFs 

developed using the EB 

technique to identify highpriority locations is a moresophisticated crash analysis 

approach that is more

accurate than using crash 

frequencies or crash rates

because they account for the 

effect of regression to the 

Figure 1: IDOT Potential for Safety Improvement Graphical Definition
HSM PART B CASE STUDY: 

SAFETY PERFORMANCE FUNCTION DEVELOPMENT

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mean. The use of SPFs also provides a rigorous analytical approach that is objective and 

consistent. The development and use of SPFs has helped the agency to shift the analysis 

approach from all crashes to fatal and severe crashes, from basic to more advanced analytical 

methods, and from congested roadways to a broader range of roadways. The modified process 

helped to allocate limited safety dollars to areas with the greatest need as a result the number of 

fatal and severe crashes in Illinois have been reduced significantly and Illinois roadways have 

the lowest number of annual fatalities since 1921.

Contact

David Piper, P.E.

Illinois Department of Transportation

Phone: (217) 558-1793

E-mail: David.Piper@illinois.gov