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Role of SPFs in SafetyAnalyst
Ray KrammesFederal Highway Administration
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FHWA Tools Supporting Implementation of the HSM
July 29 and 30, 2009 2
HSM Part Supporting Tool
B: Roadway Safety Management Process
SafetyAnalyst
C: Predictive Methods
IHSDM
D: Accident Modification Factors
CRF/AMF Clearinghouse
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Relationship among HSM, SafetyAnalyst and IHSDM
July 29 and 30, 2009 3
HSM Level of Analysis
Tool
Part B – Roadway Safety Management
Process
System-Wide(Network-Level)
SafetyAnalyst
Part C – Predictive Methods
Specific Roadway(Project-Level)
IHSDM
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SafetyAnalyst
• Analytical tool to support safety management decision making by State and local highway agencies
• Automates many of the best statistical approaches described in the Highway Safety Manual (Part B)
• Integrates all parts of the safety management process in a single, modular software package
4July 29 and 30, 2009
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Relationship between the HSM and SafetyAnalyst
July 29 and 30, 2009 5
HSM Chapter SafetyAnalyst Module 4 Network Screening 1 Network Screening 5 Diagnosis 6 Select Countermeasures
2 Diagnosis and Countermeasure Selection
7 Economic Appraisal 8 Prioritize Projects
3 Economic Appraisal and Priority Ranking
9 Safety Effectiveness Evaluation 4 Countermeasure Evaluation
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Use of SPFs in SafetyAnalyst• SafetyAnalyst methods are based on
expected accident frequency at sites• Expected accident frequency at a
site is a weighted average of observed and predicted accident frequencies
• Weights based on Empirical-Bayes methods
• Predicted accident frequencies are based on SPFs
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SPFs in SafetyAnalyst
• Negative Binomial Regression models • SPFs for 45 site subtypes:– 17 subtypes of roadway segments– 12 subtypes of intersections– 16 subtypes of ramps
• Based on HSIS data from 4 States– Default SPFs are a function of ADT only– Developed for Total and F & I accidents– Calibration procedures available to account for
local conditions– User-defined SPFs may be input
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Roadway Segment SPFs in SafetyAnalystRURAL• Two-lane• Multilane
– undivided– divided
• Freeways:– Within interchange area
• 4 lanes• 6+ lanes
– Between interchanges• 4 lanes• 6+ lanes
URBAN• Arterials
– Two-lane– Multilane
• undivided• divided
– One-way• Freeways:
– Within interchange area• 4 lanes• 6 lanes• 8+ lanes
– Between interchanges• 4 lanes• 6 lanes• 8+ lanes
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Intersection SPFs in SafetyAnalystRURAL• 3-leg with:
– minor-road STOP control– all-way STOP control– signal control
• 4-leg with: – minor-road STOP control– all-way STOP control– signal control
URBAN• 3-leg with:
– minor-road STOP control– all-way STOP control– signal control
• 4-leg with: – minor-road STOP control– all-way STOP control– signal control
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Ramp SPFs in SafetyAnalystRURAL• Diamond
– off-ramp– on-ramp
• Parclo loop – off-ramp– on-ramp
• Free-flow loop – off-ramp– on-ramp
• Free-flow outer connection • Direct or semidirect
connection
URBAN• Diamond
– off-ramp– on-ramp
• Parclo loop – off-ramp– on-ramp
• Free-flow loop – off-ramp– on-ramp
• Free-flow outer connection • Direct or semidirect
connection
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SafetyAnalystData Requirements
• Site characteristics (i.e., inventory data)– Roadway segments– Intersections– Ramps
• Accidents
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Roadway SegmentInventory Data• Segment number• Segment location
(linkable to accident data)
• Segment length (mi)• Area type
(rural/urban)• Number of through
traffic lanes (by direction)
• Presence of median (divided/undivided)
• Direction of travel (for divided highways if each direction is treated as a separate segment)
• Access control (freeway/nonfreeway)
• ADT (veh/day)• Within interchange
area? (freeways only)
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IntersectionInventory Data• Intersection
number• Intersection
location (linkable to accident data)
• Intersection location data (minor road)
• Area type (rural/urban)
• Number of intersection legs
• Type of traffic control
• Major-road ADT (veh/day)
• Minor-road ADT (veh/day)
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RampInventory Data• Ramp number• Ramp location
(linkable to accident data)
• Ramp length (mi)• Area type
(rural/urban)
• Ramp type (off-ramp/on-ramp/ freeway-to-freeway ramp)
• Ramp configuration (diamond/loop/etc.)
• Ramp ADT (veh/day)
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Accident Data
ACCIDENT-LEVEL DATA• Accident case number• Accident location
(linkable to site data)• Accident date
(day/month/year)• Relationship to
junction• Accident type and
manner of collision• Accident severity level
• Roadway segment, intersection, or ramp number
• Divided highway (side of road indicator)
VEHICLE-LEVEL DATA• Initial direction of travel• Vehicle
maneuver/action
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For More Information about SafetyAnalyst• Go to:
www.safetyanalyst.orgwww.aashtoware.org
• Contact:– Ray Krammes @ [email protected], (202) 493-3312– Vicki Schofield @ [email protected],(202) 624-XXXX
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SafetyAnalyst Modules
Module 1 – Network ScreeningModule 2 – Diagnosis and Countermeasure
SelectionModule 3 – Economic Appraisal and Priority
RankingModule 4 – Countermeasure Evaluation
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Module 1 – Network Screening
• Review highway network (or any portion of the network) to identify sites with potential for safety improvement
• Identify sites that are candidates for further investigation– Identification does not necessarily imply that
the site has an existing safety problem or has more accidents than expected
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Module 1Types of Network Screening
• Basic network screening for sites with high accident frequencies (total accidents or specific severity levels or collision types):– with peak searching on roadway segments– with sliding window on roadway segments
• High proportion of specific accident type• Sudden increase in mean accident frequency• Steady increase in mean accident frequency• Corridor screening (extended roadway sections)
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Module 2 – Diagnosis and Countermeasure Selection
• Guide user in the diagnosis of safety problems at specific sites
• Suggest array of countermeasures that address identified accident patterns
• User selects recommended countermeasures for further economic evaluation in Module 3
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Module 2Diagnosis Tools
• Collision diagrams– Provides simple collision diagram capabilities– Third-party software can be linked
• Accident summary statistics– Generates table, bar-charts, and/or pie-charts
for range of accident data elements
• Statistical tests– Test for minimum accident frequencies– Test for high proportions of accidents
• Diagnosis review questions
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Module 3 - Economic Appraisal & Priority Ranking
• Perform economic analysis of alternative countermeasures for a specific site
• Perform economic analysis of countermeasures across selected sites
• Develop priority ranking of alternative improvements
• Select an optimal mix of sites and countermeasures
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Module 3Appraisal & Ranking Measures
• Cost effectiveness• EPDO-based cost effectiveness• Benefit-cost ratio• Net benefits• Construction costs• Safety benefits• Number of total accidents reduced• Number of FI accidents reduced• Number of FS accidents reduced
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Module 4 – Countermeasure Evaluation• Determine safety effectiveness (percent
reduction in crashes) for specific implemented countermeasures
• Conduct before-after evaluation of crash frequencies using the Empirical Bayes (EB) approach
• Conduct before-after evaluation of shifts in crash type proportions
• Reliable results require multiple sites and multiple years of before and after data for each site
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