DEVELOPING AND USING STATE SPECIFIC SPFS IN VIRGINIA 

	 	 	 	 	

i

-

Roadway Safety Data Program 

DEVELOPING

PERFORMANCE FUNCTIONS (SPFS) IN VIRGINIA

ROADWAY SAFETY DATA AND ANALYSIS

CASE STUDY  

FHWA‐SA‐14‐078

Prepared for  

Federal Highway Administration

AND USING STATE‐SPECIFIC SAFETY

Office of Safety

Roadway Safety Data Program

http://safety.fhwa.dot.gov/rsdp/

April 13, 2015  
 

 

 

 

 

 

 

 

DEVELOPING AND USING STATE-SPECIFIC SPFS IN VIRGINIA 

Notice 

This document is disseminated under the sponsorship of the U.S. Department of Transportation 

in the interest of information exchange. The U.S. Government assumes no liability for the use of 

the information contained in this document. 

The U.S. Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear in this report only because they are considered essential to the 

objective of the document. 

Quality Assurance Statement 

The Federal Highway Administration (FHWA) provides high-quality information to serve 

Government, industry, and the public in a manner that promotes public understanding. 

Standards and policies are used to ensure and maximize the quality, objectivity, utility, and 

integrity of its information. FHWA periodically reviews quality issues and adjusts its programs 

and processes to ensure continuous quality improvement. 

i
DEVELOPING AND USING STATE-SPECIFIC SPFS IN VIRGINIA 

ACKNOWLEDGEMENTS 

The Roadway Safety Data Program team would like to thank the following individuals who 

graciously provided information needed to develop this case study. 

Name Title Department Email/Phone#

 Anne Booker Assistant Regional 

Traffic Engineer Virginia DOT 

anne.booker@vdot.virginia.gov 

504.375.0143

 Mark Cole 

Assistant Division 

Administrator – Highway 

Safety 

 Virginia DOT mark.cole@vdot.virginia.gov 

804.786.4196

 Jeff Kuttesch Richmond Area Traffic 

Engineer Virginia DOT jeffrey.kuttesch@vdot.virginia.gov 

804.524.6114

 Young-Jun Kweon Senior Research Scientist Virginia DOT young-jun.kweon@vdot.virginia.gov 

434.293.1949

 In-Kyu Lim HSIP Data Analysis Team 

Leader Virginia DOT in-kyu.lim@vdot.virginia.gov 

804.786.3887

 Bob Rasmussen Program Administration 

Manager III Virginia DOT robert.rasmussen@vdot.virginia.gov 

804.786.6219

 Virginia Department of Transportation 

 Traffic Engineering Division 

1401 East Broad Street 

 Richmond, VA 23219 

ii 
DEVELOPING AND USING STATE-SPECIFIC SPFS IN VIRGINIA 

TABLE OF CONTENTS 

INTRODUCTION .........................................................................................................1
 

BACKGROUND ............................................................................................................1
 

OBJECTIVE ....................................................................................................................2
 

AUDIENCE ....................................................................................................................3
 

SPF USE IN SAFETY DECISIONMAKING................................................................3
 

GAINING INTERNAL/LEADERSHIP’S SUPPORT ..................................................4
 

RESULTS........................................................................................................................ 5
 

RESULTS SUMMARY ...................................................................................................5
 

Funding ......................................................................................................................... 6
 

Benefits......................................................................................................................... 6
 

Barriers and How They Were Overcome................................................................6
 

Lessons Learned ..........................................................................................................6
 

Next Steps....................................................................................................................7
 

AGENCY CHARACTERISTICS ..................................................................................7
 

Organizational Structure ...........................................................................................7
 

Personnel......................................................................................................................8
 

REFERENCES ................................................................................................................8
 

AGENCY CONTACT INFORMATION .....................................................................8
 

iii 
DEVELOPING AND USING STATE-SPECIFIC SPFS IN VIRGINIA 

List of Figures 

Figure 1 Timeline of SPF development................................................................................ 2 

Figure 2 Slide used by SPF development team to explain the SPF methodology........... 4 

Figure 3 Slide used by SPF development team to explain the PSI methodology............ 5 

iv 
DEVELOPING AND USING STATE-SPECIFIC SPFS IN VIRGINIA 

ACRONYMS 

EB Empirical Bayes 

FTE Full-Time Equivalent 

HSIP Highway Safety Improvement Program 

HSM Highway Safety Manual 

PSI Potential for Safety Improvement 

SPF Safety Performance Function 

TSN Target Safety Needs 

VCTIR Virginia Center for Transportation Innovation and Research 

VDOT Virginia Department of Transportation 

v 
DEVELOPING AND USING STATE-SPECIFIC SPFS IN VIRGINIA 

EXECUTIVE SUMMARY 

The Virginia Department of Transportation (VDOT) emphasizes data-driven decision-making 

and desires to improve safety and safety data. From this desire, VDOT implemented a 

comprehensive set of State-specific Safety Performance Functions (SPFs) covering 98 percent of its 

State-maintained roadway locations. The impetus for VDOT developing their own SPFs and 

analytical tools arose from the decision that AASHTOWare Safety AnalystTM did not meet their 

needs. VDOT developed State-specific SPFs using historical crash, traffic, and roadway inventory 

data. SPF developers worked closely with engineers throughout the development process to see 

whether each SPF was implementable for all types of improvements (spot, corridor, and systemic). 

To date, VDOT has developed 24 SPFs covering a majority of roadway facilities, including two-lane 

roads, intersections, and freeways/multi-lane highways. 

After performing network screening, the VDOT central office identifies the top 100 sections and 

top 100 miles of segments with the largest Potential for Safety Improvement (PSI). The list is then 

sent to the district engineers who determine which sites to prioritize based on practical experience 

and knowledge of their area. VDOT has noted several benefits of Virginia’s SPF implementation 

effort. For example, the advanced data-driven process leads to better use of funds, benefits for both 

systemic and spot improvements are quantifiable, VDOT can better manage public concerns, and 

VDOT can compare locations to prioritize projects. The SPF development team conducts training 

(including an annual “roadshow” to all nine districts) and hosts webinars to ensure district engineers 

understand the methodology and how to use the SPFs. VDOT has not mandated the use of SPFs 

and PSIs by the districts because the process of introducing a new methodology takes time, but the 

district engineers know it is the preferred method for network screening. 

vi 
DEVELOPING AND USING STATE-SPECIFIC SPFS IN VIRGINIA 

INTRODUCTION 

The Virginia Department of Transportation (VDOT) emphasizes data-driven decision-making 

and makes safety data improvement a focus of continuous effort and long-term planning. From 

this desire to improve safety and safety data, Virginia implemented a comprehensive set of 

State-specific Safety Performance Functions (SPFs) covering 98 percent of the State-maintained 

roadway locations in the Commonwealth. An SPF is a mathematical relationship between crash 

frequency and site characteristics such as traffic volume, segment length, and other roadway 

attributes. VDOT developed State-specific SPFs using historical Virginia crash, traffic, and 

roadway inventory data separately for each grouping of roadway locations based on 

homogeneous roadway characteristics. The SPFs are statistical functional forms and estimate 

the average crash frequency for a specific location by traffic volume and segment lengths. Actual 

crash frequency for any location can be compared to the SPF for locations of that type to see if, 

for the level of traffic volume, the location has more than the predicted number of crashes. 

VDOT incorporates the comparisons of actual- to predicted-crash frequencies in its network 

screening and provides district engineers with a list of the top 100 intersections and top 100 

miles of roadway segments drawn from those locations that are above the SPF. In other words, 

these locations have more crashes than other similar locations for the same level of traffic 

volume. VDOT has developed algorithms capable of screening the entire roadway network and 

has recently begun implementing the algorithms for prioritizing Virginia’s roadways for safety 

improvement. 

The purpose of this case study is to describe the successes and ongoing challenges related to 

developing and implementing a comprehensive set of State-specific SPFs in Virginia. 

BACKGROUND 

VDOT maintains 58,430 centerline miles of roadway, which comprises approximately 78 

percent of the 74,748 centerline miles of public roadway in Virginia. VDOT also maintains data 

in the Roadway Inventory for 85,824 (95 percent) of the 90,614 intersections in the 

Commonwealth. 

Staff from Virginia Center for Transportation Innovation and Research (VCTIR)—a joint 

program of VDOT and the University of Virginia—and the VDOT Traffic Engineering Division 

worked together to develop the Virginia-specific SPFs and conduct training for VDOT District 

engineers. 

Prior to the Virginia-specific SPF development effort, VDOT purchased a license for 

AASHTOWare Safety AnalystTM in 2009, but subsequently decided that the software did not 

meet their needs. VDOT differs from most State transportation departments because it 

1
 
DEVELOPING AND USING STATE-SPECIFIC SPFS IN VIRGINIA 

maintains such a large percentage of the roadway miles, including secondary roads. 

Consequently, the roadway database is extensive. To manage this network, VDOT divides the 

Commonwealth into nine districts. Using Northern Virginia as a test district, VDOT identified 

9,000 errors in the AASHTOWare Safety AnalystTM records that would have required manual 

review in order to correct the problems. This was the impetus for VDOT to develop their own 

SPFs and analytic tools. 

VDOT developed SPFs using a standard methodology in which staff would collect data, filter the 

data by facility types and urban/rural designation, then develop the SPFs for specific categories 

of roadway. VDOT, VCTIR, and Traffic Engineering staff worked together from the beginning of 

the SPF development program. Whenever the district engineers asked to implement the SPFs 

for a special purpose/analysis (including spot, corridor, and systemic improvements), the 

development team and the engineer determined if the SPF was implementable—meaning it 

could be used in all types of improvements. If VDOT could not use the SPF in a systemic 

fashion, the SPF was not used. To date, the project has resulted in 24 SPFs covering the 

majority of roadway locations managed by VDOT. Figure 1 shows which SPFs were developed 

first through 2014. 

Figure 1. Timeline of SPF development. 

Through this process, VDOT found that there were fewer obstacles advancing the use of SPFs 

when decision-makers understand the development process and how SPFs are applied. VDOT 

continues to strive for all districts to use Virginia-specific SPFs as the primary resource to 

develop Highway Safety Improvement Projects (HSIP) in Virginia. 

OBJECTIVE 

Virginia’s SPF implementation effort strives to: 

 Identify intersections and segments that have the most potential for improvement. 

 Allow district engineers to select appropriate project locations by advanced screening 

results. 

2
 
DEVELOPING AND USING STATE-SPECIFIC SPFS IN VIRGINIA 

 Select and prioritize projects with more efficiency and therefore, better allocate funds. 

 Support before-after analysis to quantify the safety benefit once projects are 

completed. 

AUDIENCE 

This case study may be of interest to the following audiences: 

	 State Department of Transportation: Safety Engineering, Design, Planning, Maintenance, 

GIS, and Asset Management Units. 

	 Local and Regional: City and County Public Works/Engineering/Transportation 

Departments, Metropolitan Planning Organizations, and Regional Planning Commissions. 

	 Local Technical Assistance Programs. 

	 Consultants and private industries involved in safety. 

SPF USE IN SAFETY DECISIONMAKING 

VDOT developed their SPFs using the five years of historical crash data (2004-2008). For each 

facility/location type, VDOT developed two separate SPFs: one for total crashes and the other 

for fatal+injury crashes. VDOT developed SPFs for the following types of roadway facilities: 

	 Two-lane roads: Two types (rural and urban segments) 

	 Intersections: Eight types (rural/urban; three-leg/four-leg; minor-road stop control/signal 

control intersections) 

	 Multilane highways: Four types (rural/urban; undivided/divided segments) 

	 Freeways: Ten types (rural basic-area/interchange-area segments with four/six+ lanes; 

urban basic-area/interchange area segments with four/six/eight+ lanes) 

In network screening, VDOT uses the empirical Bayes (EB) Method to estimate the expected 

long-term crash frequency at a site. In 2013 and 2014, VDOT used the most recent three years 

of crash data (2010-2012) to calculate the Potential for Safety Improvement (PSI). The PSI is the 

expected number of crashes for the site minus the predicted number of crashes based on the 

SPF for that facility type. A site with a positive PSI warrants examination and those with the 

highest PSI values should be considered high priority. As with SPFs, VDOT calculates the PSI for 

total crashes and fatal+injury crashes for each site. The most recent three years (2010-2012) of 

data were used for calculating PSIs for total crashes and fatal+ injury crashes. Sites with high 

value in at least two years for both total and fatal+injury PSIs were selected for consideration 

for Virginia’s Highway Safety Improvement Program (HSIP). Those sites were called Target 

Safety Need (TSN) sites. VDOT identified 2,358 intersections and 2,744 miles of segments as 

TSN sites for Virginia’s HSIP. All nine districts in Virginia have undertaken numerous projects 

3
 
 

DEVELOPING AND USING STATE-SPECIFIC SPFS IN VIRGINIA 

from the list of TSN sites, most of them being systemic in nature (i.e., low-cost treatments 

applied across multiple sites with similar characteristics). 

GAINING INTERNAL/LEADERSHIP’S SUPPORT 

To gain executive support for the new SPF methodology, the development team met with many 

high-ranking staff in VDOT, where the development team fielded many questions about the 

methodology. Those ranking above the State traffic engineer (i.e., the chief engineer level) were 

largely concerned that traffic engineers had an effective method for picking the most suitable 

projects and prioritizing those projects. The team used various graphics (e.g., graphs, tables, 

formulae, maps) when explaining the methodology, which was an effective strategy for gaining 

buy-in from the high-ranking staff with limited knowledge or experience with highway safety. 

For example, the team used slides, such as Figure 2 and Figure 3, to explain the methodology. 

Figure 2. Example of a slide used by the SPF developers. 

4
 
DEVELOPING AND USING STATE-SPECIFIC SPFS IN VIRGINIA 

Figure 3. Example of a slide used by the SPF developers. 

After gaining executive support, the SPF development team met with the district engineers to 

demonstrate how the methodology works and how the data can be used. At first, the SPF 

methodology was confusing for many of the district engineers, but several webinars and training 

sessions have assisted the engineers become more comfortable with using SPFs. 

RESULTS 

After performing the network screening and determining the PSI values, the central office 

identifies the top 100 sections and top 100 miles of segments with the largest PSI. The list is 

then disseminated to the district engineers who then determine which sites to prioritize based 

on practical experience and knowledge in the area. 

RESULTS SUMMARY 

The Highway Safety Administrator was responsible for promoting the program to executive 

level staff internally, which included conversations and presentations demonstrating the 

following SPFs process and benefits. 

5
 
DEVELOPING AND USING STATE-SPECIFIC SPFS IN VIRGINIA 

Funding 

The budget for developing the SPFs is built into staff members’ salary and comes from the 

VDOT budget. The cost to implement and maintain the program is approximately equivalent to 

a full-time engineering position. 

Benefits 

VDOT identified the following benefits of Virginia’s SPF Implementation: 

 Advanced data-driven process leads to better use of VDOT funds.
 

 Potential benefits from both systemic and spot improvements are quantifiable. 

 Districts can better prioritize public concerns by using crash data to determine if 

treatments are warranted. 

 Districts can compare intersections or segments with similar characteristics to help 

prioritize projects. 

	 Districts can use PSI information to identify characteristics of safer intersections or 

segments that may be implemented as countermeasures at other locations with higher PSI 

values. 

	 District Staff have a better understanding of SPFs and PSI through training from central 

office staff 

Barriers and How They Were Overcome 

New strategies and technologies are often met with resistance and VDOT found that many 

district engineers were initially unsure of how to apply SPFs. The SPF development team 

overcame this barrier by conducting numerous training sessions and webinars. The team also 

established an annual “roadshow,” in which they spend 10 days a year traveling to each of the 

nine districts to discuss new safety data, review HSIP projects, go through district-level data and 

the network screening process, and present information on how to fund new safety projects. 

This annual meeting gives district engineers an opportunity to discuss what they learned using 

SPFs and how SPFs can be used to support decision making in the future. 

Lessons Learned 

VDOT’s advice to other States interested in developing State-specific SPFs improving their 

safety decision-making process is to start slow and be patient. Project staff should expect some 

reluctance on the part of decision makers adopting new, unfamiliar methodologies. In Virginia, 

the SPF developers took the time to work with engineers to ensure they fully understood the 

concepts and how to use the SPFs and PSIs to make better decisions, which in turn made the 

engineers more accepting of the new practice. 

6
 
DEVELOPING AND USING STATE-SPECIFIC SPFS IN VIRGINIA 

Next Steps 

VDOT has not yet mandated the use of SPFs and PSIs by the Districts. The district engineers 

know that using SPFs is the preferred method for network screening, but VDOT managers 

understand that switching to the new method is a process and will take time. VDOT has 

included an item in its business plan that ensures safety funding is given to highest priority 

locations using a standardized process in which the central office performs the network 

screening, identifies locations for improvement, and then develops a way to measure the 

effectiveness of the treatment. 

Beginning this year, VDOT will adopt a “benchmarking” approach to identify sites with negative 

PSI values (i.e., sites where less crashes occur than are expected). VDOT will compare these 

locations with sites with similar characteristics, but have a positive PSI, to determine what 

makes those sites safer. 

VDOT learned through a pilot study that HSM SPFs are quite different than the State-specific 

SPFs, which has been challenging for the district engineers. To address this issue, VDOT plans 

to launch a project to customize the HSM and conduct trainings for the districts. This training 

will include State-specific examples using the analytic methods described in the HSM. 

In mid-2015, VDOT plans to launch new software that will allow users to search crash data and 

customize outputs. Users will be able to look at crashes that were associated with different 

variables (e.g., alcohol, speed, and belted) at a high level. This will allow users to only include 

relevant variables, which lead to better informed, data-driven decisions. VDOT has also planned 

to develop SPFs for roadway departure crashes. 

AGENCY CHARACTERISTICS 

Organizational Structure 

For the purposes of this case study, VDOT’s central staff consists of the Traffic Engineering 

Division and Executive Staff. These are the practitioners and leaders who, respectively, 

promoted adoption of HSM-style analytic methods and approved those methods as a part of 

Departmental policy going forward. The Traffic Engineering Division staff collaborated with the 

VCTIR researchers to direct development of the Virginia-specific SPFs and the PSI calculations, 

promote their use by district engineers, and incorporate the analyses into the Department’s 

network screening and project selection procedures. District engineers are responsible for 

selecting and justifying safety projects in their geographic areas. They have the option to select 

projects using methods other than strict adherence to the PSI-based prioritizations; however, 

they must be able to justify their decisions, preferably using a data-driven approach. 

7
 
 

DEVELOPING AND USING STATE-SPECIFIC SPFS IN VIRGINIA 

Personnel 

The SPF development team worked on the effort as part of their normal duties and did not 

require assistance from contractors or other outside help. The combined effort to support 

Virginia’s SPF Implementation is equal to having one full-time equivalent (FTE) staff member. 

REFERENCES 

The following resources were consulted in development of this case study: 

1. Mark Cole and Bob Rasmussen. Telephone Interview. December 15, 2014 

2. In-Kyu Lim and Young-Jun Kweon. Telephone Interview. December 15, 2014 

3. Anne Booker. Telephone Interview. December 18, 2014 

4. Jeff Kuttesch. Telephone Interview. December 18, 2014 

AGENCY CONTACT INFORMATION 

Anne Booker 

Assistant Regional Traffic Engineer 

Southwest Region Operations 

Virginia Department of Transportation 

1596 Deorah Lane 

Salem, VA 24153 

Anne.Booker@vdot.virginia.gov 

540.375.0143 

Mark Cole 

Assistant Division Administrator – Highway Safety 

Traffic Engineering Division 

Virginia Department of Transportation 

1401 East Broad Street 

Richmond, VA 23219 

Mark.Cole@vdot.virginia.gov 

804.786.4196 

Jeff Kuttesch 

Richmond Area Traffic Engineer 

Central Region Operations 

Virginia Department of Transportation 

2201 West Hundred Road 

Chester, VA 23831 

Jeffrey.Kuttesch@vdot.virginia.gov 

8
 
 

 

DEVELOPING AND USING STATE-SPECIFIC SPFS IN VIRGINIA 

804.524.6114 

Young-Jun Kweon 

Senior Research Scientist 

Virginia Center for Transportation Innovation and Research (VCTIR) 

Virginia Department of Transportation 

530 Edgemont Road 

Charlottesville, VA 22903 

Young-Jun.Kweon@vdot.virginia.gov 

434.293.1949 

In-Kyu Lim 

HSIP Data Analysis Team Leader 

Traffic Engineering Division 

Virginia Department of Transportation 

1401 East Broad Street 

Richmond, VA 23219 

In-Kyu.Lim@vdot.virginia.gov 

804.786.3887 

Bob Rasmussen 

Program Administration Manager II 

Traffic Engineering Division 

Virginia Department of Transportation 

1401 East Broad Street 

Richmond, VA 23219 

Robert.Rasmussen@vdot.virginia.gov 

804.786.6219 

9