Implementing Safety Performance Functions in Virginia. Part I : SPF Fundamentals BACKGROUND OUTLINE. BASICS: What? BASICS: When & How?

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1 BACKGROUND Implementing Safety Performance Functions in Virginia June 9, VDOT Safety Workshop VDOT has been applying its efforts to develop and implement Virginia-specific s (VA s) for network screening to help identify and prioritize locations needing highway safety improvements. VDOT has completed the development of VA s for various facility types that cover about 9% of the, miles of public roads in Virginia. Using the VA s, VDOT has developed algorithms capable of screening the entire roadway network and has recently begun implementing the developed algorithms for prioritizing Virginia s roadways for safety improvement. Virginia is one of the first states in the country to apply state-specific s across a statewide road network consisting of various facility types. This workshop will provide background information regarding development and will showcase the implementation results of VDOT s most recent network screening. OUTLINE Basics VA s EB & PSI PSI Calculation PSI NS TSN Young-Jun Kweon Senior Research Scientist VCTIR In-Kyu Lim HSIP Data Analysis Team Leader Part I I : Fundamentals Young-Jun Kweon Senior Research Scientist VCTIR BASICS: What, When, How, Why? BASICS: Types BASICS: Ways VA : Overview VA : Development EB & PSI: Introduction BASICS: What? BASICS: When & How? What is? Safety Performance Function () is a mathematical relationship between crash frequency and some site characteristics (e.g., AADT and segment length). Typical forms: Segment Intersection # # (e.g.) Segment # When & how to use it? To predict # of crashes of a site under given conditions (e.g., AADTs, Length) With the prediction, we can assess safety of the site under the conditions (e.g.) Segment # Sites above the are potentially unsafe.

2 BASICS: Why? BASICS: Why? Why do we need s? National safety initiatives and tools are based on s (e.g., SafetyAnalyst, HSM). Traditional measures (e.g., frequency and rate) have shortcomings. s can overcome these shortcomings. How do we measure safety of a site? Crash frequency (e.g.) #crash/year Crash rate (e.g.) #crash/aadt (e.g.) #crash=f (AADT) = Crash Frequency #Crashes AADT 7 AADT BASICS: Types BASICS: Ways types of (by purpose or use) ways of customizing s Program-Level Mainly to screen a large road network to find potentially unsafe sites (e.g., Safety Analyst s s) All inputs for s should be readily available in a file or database Typical inputs are AADT and segment length (e.g.) For rural multilane segments, # Factors other than AADT, length, urban/rural, and # of lanes are assumed to be fixed. Project-Level Mainly to evaluate a few design alternatives of a single site (e.g., HSM s s) Some inputs are already available and others need to be collected. Several additional factors beyond AADT and length are accounted for. (e.g.) For rural multilane segments, # Other factors such as lane width, shoulder width, and lighting condition are accounted for by CMFs. Calibrate s Default s developed based on national data are available. These s can be used for our state after some adjustments. Adjustments can be done by calibration factor Default Default C Develop s Default s adjusted by calibration factors may not be suited for our state. Development of new s using local data is needed. Even with new s, calibration factors are still needed for different years/districts. Default New Default C 9 /9/ VA : Overview VA : Development Virginia-specific Safety Performance Function (VA ) VDOT developed VA s for the past years (9-) to help identify and prioritize locations needing safety improvements. VA s were developed for total crashes and fatal & injury crashes, separately. VA s were developed for a total of types of roadway facilities: o Two-lane roads: types (rural and urban segments) o Intersections: types (rural/urban, -leg/-leg, minor-road stop control/signal control intersections) o Multilane highways: types (rural/urban, undivided/divided segments) o Freeways: types (rural basic-area/interchange-area segments with /+ lanes; urban basic-area/interchange area segments with //+ lanes) The developed s cover about 9% of, miles of roads maintained by VDOT. Total Crashes for Multilane Highways Site Site Subtype Description Subtype Code Rural multilane undivided highway segments -.. Rural multilane divided highway segments -.. Urban multilane undivided arterial segments -..9 Urban multilane divided arterial segments -.9. #

3 VA : Development VA : Development Total Crashes for Multilane Highways: VA s vs. Default s Predicted # of Crashes (per mile) Predicted # of Crashes (per mile) 7 Rural Undivided 9 Rural Divided C=. for North Carolina C=.7 for Minnesota 7 AADT in, AADT in, Urban Undivided Urban Divided C=. for North Carolina C=.97 for North Carolina 9 7 AADT in, AADT in, Total Crashes for Freeways: Site Site Subtype Description Subtype Code Rural freeway segments lanes Rural freeway segments + lanes Rural freeway segments within an interchange area lanes Rural freeway segments within an interchange area + lanes -.7. Urban freeway segments lanes -.7. Urban freeway segments lanes Urban freeway segments + lanes Urban freeway segments within an interchange area lanes Urban freeway segments within an interchange area lanes -.. Urban freeway segments within an interchange area + lanes # VA : Development EB & PSI: Introduction Total Crashes for Freeways: VA s vs. Default s Predicted # of Crashes (per mile) Predicted # of Crashes (per mile) Rural Lanes C=.97 for Minnesota Urban Lanes Predicted # of Crashes (per mile) Rural + Lanes C=. for California AADT in, AADT in, Urban Lanes Urban Lanes C=.9 for Washington C=. for Washington 7 AADT in, 7 AADT in, AADT in, Empirical Bayes (EB) Method The EB method is a statistical technique to estimate the expected long-term crash frequency of a site. It combines the observed crash frequency with the crash frequency predicted by the. Predicted # of crashes = Prediction It resolves some critical pitfalls that an alone can create such as the regressionto-the-mean bias. Potential for Safety Improvement (PSI) +PSI Observed # of crashes =Actual crash frequency If a site has +PSI, it is unsafe and has a potential for safety improvement. PSI Closing Part I I : Fundamentals BASICS: What, When, How, Why? BASICS: Types (Program-Level vs. Project-Level) BASICS: Ways (Calibrate vs. Develop) VA : Overview VA : Development (VA s vs. Default s) EB & PSI : Introduction Part II I : Fundamentals Implementation In-Kyu Lim HSIP Data Analysis Team Leader OVERVIEW PSI CALCULATION PSI NS: Total Crashes PSI NS: Fat+Inj Crashes TSN: Introduction TSN: VA PSI Matrix TSN: Results 7

4 OVERVIEW PSI CALCULATION The most recent years ( ) of data were used for calculating PSIs for total crashes and fatal & injury crashes, separately. The network screening algorithm based on the PSIs, called PSI NS, was executed on intersections and segments, separately. Over 9, sites (, segments+9, intersections) were screened. Target Safety Need (TSN) sites were further identified among sites with PSI >. In the end,, intersections and,7 miles of segments were identified as TSN sites for VA HSIP. PSI = N Expected - N Predicted + PSI - 9 PSI NS: Total Crashes PSI NS: Total Crashes 9,.9 miles,9. miles,9. miles SUM,. miles Zoom in on Highland County PSI NS: Total Crashes PSI NS: Total Crashes Highland County ( ) years years year Statewide.. years years year

5 PSI NS: Fat+Inj Crashes PSI NS: Fat+Inj Crashes,7.7 miles,.7 miles,79.7 miles SUM 9,. miles Zoom in on Highland County PSI NS: Fat+Inj Crashes PSI NS: Fat+Inj Crashes Highland County ( ) years years year Statewide.. years years year 7 TSN: Introduction Screening results of multiple years need to be combined for project development of HSIP. The PSI NS algorithm was also applied for intersections. A matrix with Total and Fatal & Injury Crash PSIs of multiple years was created to combine the screening results, called Virginia PSI Matrix. Separate matrices were prepared for segments and intersections. A site being identified in at least years in both Total and Fatal & Injury Crash PSIs was selected for consideration for HSIP, called Target Safety Need (TSN) site. 9

6 TSN: VA PSI Matrix TSN: VA PSI Matrix S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, TSN Sites TSN: Results TSN: Results Segments (miles) Fatal and Injury Crash PSI > Not Appear TOTAL Not Appear, ,997.7,9.9, ,77.9 Total Crash PSI >,.,9.,9.9 7.,.....,9. TOTAL 7,.,.,. 7.9,. TSN for VA HSIP Intersections (# of sites) Fatal and Injury Crash PSI > Not Appear TOTAL Not Appear, 7,7,,9 9,7 Total Crash PSI > 7,99,, 9 7 7, TOTAL,9,97,,7 Intersections District # of Intersection Unknown 9 Bristol Salem Lynchburg Richmond 7 Hampton Roads Fredericksburg 7 Culpeper Staunton A NOVA TOTAL, Segments District Length (mile) Unknown.9 Bristol.97 Salem. Lynchburg 9.97 Richmond 7.7 Hampton Roads 9. Fredericksburg. 7 Culpeper. Staunton 7. A NOVA. TOTAL,7. Closing uploaded lists of TSN intersections and segments by District at CO Safety Team Site. is developing Six Year Safety Plan based on - TSN results. will update the TSN results when crash data is ready. HSIP Tracy Turpin HSIP Manager CONTACTS Mark Cole Asst. Div. Administrator-Highway Safety Young-Jun Kweon In-Kyu Lim Senior Research Scientist HSIP Data Analysis Team Leader VCTIR