FREEWAY SEGMENT SAFETY PERFORMANCE FUNCTION (SPF) DEVELOPMENT

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FREEWAY SEGMENT SAFETY PERFORMANCE

FUNCTION (SPF) DEVELOPMENT

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INTRODUCTIONS

o Scott Himes, VHB o Ian Hamilton, VHB

o Kendra Schenk, B&N

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AGENDA

o Introduction and Objectives – Scott Himes

o Data Collection and Integration – Ian Hamilton o SPF Development Approach – Scott Himes

o Project-Level Validation– Kendra Schenk

o Questions

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INTRODUCTION AND OBJECTIVES

Scott Himes

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INTRODUCTION AND OBJECTIVES

o SPFs play a critical role in reliable safety

management

o SPFs serve two primary roles

o Network screening

o Project-level safety prediction

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INTRODUCTION AND OBJECTIVES

o There are two methods for obtaining SPFs

o Calibrating existing SPFs

o Developing SPFs from jurisdiction-specific data

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INTRODUCTION AND OBJECTIVES

o Project motivations

o Ohio completed calibration of HSM project design-level models o Prioritized site types predicting

poorly

o Desire for directional prediction o Counterintuitive relationships

when adding lanes in some

circumstances after calibration

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INTRODUCTION AND OBJECTIVES

o Project Objectives

o Develop planning-level SPFs for ODOT freeways

o Develop bi-directional and directional project design-level SPFs consistent with those found in the HSM

o Evaluate project design-level SPFs along with HSM CMFs for performance

o Suggest updates to ODOT’s ECAT to incorporate recommended project design-level SPFs

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DATA COLLECTION &

INTEGRATION

Ian Hamilton, AICP

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DATA COLLECTION – STUDY AREA o Collected data for

several freeways.

o Interstates 70, 71, 75, 77, 80, and 90.

o State Routes 2 and 11.

o Combination of manual location and linear

referencing techniques.

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DATA COLLECTION – EXISTING INVENTORIES

o Data already available through ODOT’s comprehensive set of road and traffic data.

o Through lanes

o Median type and width

o Shoulder widths (inside/outside) o Annual Average Daily Traffic (AADT) o Ramp locations

o Supplementary datasets support nuanced differences in different segments.

o Barriers (inside/outside) o Lighting

o Curvature

o Additional data collected to support directional and bidirectional model development.

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DATA COLLECTION – RAMP LOCATIONS o Located ramp gores

and any associated taper starts/ends.

o Determined key characteristics:

o Speed change lane or lane add/drop.

o Left- or right-side ramp

o Flagged recent construction or atypical situations (e.g., toll plazas).

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DATA COLLECTION – FINAL DATASETS

o Created 3 datasets for model development:

o Speed change lanes.

o Directional segments.

o Bidirectional segments.

o Used ODOT’s Linear Referencing System

(LRS) to help integrate data and calculate

relative positions of data along freeways.

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DATA COLLECTION – LINEAR REFERENCING

o Linear referencing served as the basis for all segment data collection.

o Buffer and remove construction locations.

o Locate supplementary datasets on segments (e.g., barriers, curvature, and lighting).

o Locate relevant ramps (and ramp AADT).

o Locate upstream and downstream ramp gores.

o Routes dynamically segmented based on ODOT

base data to create uniform segments.

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DATA COLLECTION – LINEAR REFERENCING

o Challenge working with LRS involved county-

based routes.

o Difficult to track

upstream/downstream features across county lines.

o Reconstructed a temporary LRS to

calculate upstream/

downstream ramps and

AADT

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DATA COLLECTION – DIRECTIONAL SEGMENTS

o Directional segments developed before bidirectional segments.

o Construction during the study period and other atypical situations buffered and removed.

o Cross-section data reflect single direction of travel.

o If a segment contained an A-, B-, or C-type weave, the lower

“letter” was assigned to the segment (i.e., C over B and B over A).

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DATA COLLECTION – BIDIRECTIONAL SEGMENTS

o Bidirectional segments derived from overlapping directional segments.

o Construction, speed change lanes, or atypical situations may

remove one travel direction; these were removed from bidirectional pool.

o Data aggregated between both directions (e.g., travel lanes) or assumed based on common values.

o Upstream and downstream elements determined based on directional data.

o If a directional segment was truncated or distances changed, this value was recalculated.

o Same rules applied for determining weave sections.

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SPF DEVELOPMENT APPROACH

Scott Himes, PhD, PE

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NETWORK SCREENING SPF DEVELOPMENT

o Network screening SPFs

o Function of segment length and AADT o Impact of other factors ignored

o Model forms tested

o Power model:

o Hoerl model:

o Predict crash frequency for bi-directional

segments

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NETWORK SCREENING SPF DEVELOPMENT

o SPF development includes all segments o Freeway facility types evaluated

Area Type Lanes Segment

Type Number of

Segments Total Mileage Total

Crashes FI Crashes

Rural 4 Base 293 592.44 16,403 3,359

Interchange 331 103.05 4,112 829

6 Base 74 210.25 11,435 2,284

Interchange 92 21.84 2,056 422

Urban

4 Base 521 410.15 17,537 3,968

Interchange 1,355 359.83 33,718 8,012

6 Base 259 215.41 17,982 4,230

Interchange 930 273.90 59,836 14,517

8+ Base 81 46.96 8,001 2,073

Interchange 437 128.78 39,403 10,148

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NETWORK SCREENING SPF DEVELOPMENT

o Example SPF representation

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NETWORK SCREENING SPF DEVELOPMENT

o Cumulative residuals used for validation

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PROJECT DESIGN-LEVEL SPF DEVELOPMENT

o Project design-level SPFs

o Include additional features to improve predictions

o Can be used to evaluate the effects of changing features o Subject to data availability

o Approaches tested

o Predict crash frequency for both directions combined o Focus crash prediction on individual direction

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PROJECT DESIGN-LEVEL SPF DEVELOPMENT

o Crash types evaluated

o Multiple vehicle o Single vehicle

o Crash severity evaluated

o Fatal and injury

o Property damage only

o Individual severity evaluated using SDFs

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PROJECT DESIGN-LEVEL SPF DEVELOPMENT

o Crash prediction model

o Basic freeway segments:

o Speed change lane segments:

o Attempted to identify AFs consistent with HSM o Some AFs excluded

o Data unavailable

o Values the same for all sites (LW generally 12 ft) o No significant relationship found

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PROJECT DESIGN-LEVEL SPF DEVELOPMENT

o SPF applicability

Site Type Crash Type Crash Severity Area type Lanes

Freeway segments (fs)

Multiple vehicle

Fatal and injury Rural and urban All lanes Property damage only Rural and urban 2 lanes

Rural and urban 3+ lanes

Single vehicle

Fatal and injury Rural and urban 2 lanes Rural and urban 3+ lanes Property damage only Rural and urban 2 lanes

Rural and urban 3+lanes Entrance speed-change

lanes (en)

Combined Fatal and injury Combined 2 lanes

3+ lanes Combined Property damage only Rural and urban Combined Exit speed-change

lanes (ex)

Combined Fatal and injury Combined Combined

Combined Property damage only Combined Combined

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PROJECT DESIGN-LEVEL SPF DEVELOPMENT

o AFs and applicability

Adjustment Factor Freeway Segments Entrance Speed-

Change Lanes

Exit Speed- Change Lanes MV FI MV PDO SV FI SV PDO FI PDO FI PDO

Inside shoulder width       

Outside shoulder width  

Depressed median width        

Degree of curvature      

Median barrier    

Outside barrier  

Downstream exit lane change  

Upstream entrance lane change 

Lane addition by ramp  

Lane drop by ramp  

Type A/B weaving section  

Posted speed limit 

Left-side ramp    

Proportion curve  

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PROJECT DESIGN-LEVEL SPF DEVELOPMENT

o SDF Factors

Severity Factor Freeway Segments Speed-Change Lanes

KA Severity B Severity KA Severity B Severity

Posted speed Increase Increase Increase Increase

Outside shoulder width Increase Increase Decrease N/A

Inside shoulder width Decrease N/A N/A N/A

Median width Increase Increase N/A Decrease

Proportion outside barrier Decrease N/A N/A N/A

Average degree of curve N/A Increase Increase N/A

Urban area type Decrease Decrease Decrease Decrease

Proportion median barrier Increase Increase N/A Decrease

Presence of lighting Decrease Decrease N/A N/A

Ramp AADT N/A N/A Decrease Decrease

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PROJECT-LEVEL

VALIDATION OF MODELS

Kendra Schenk, PE, PTOE, RSP^2I

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SAMPLE PROJECTS

26 Project Sites

Rural Urban

2 Lanes 5 sites 4 sites

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PROCESS

Uncalibrated HSM

Ohio- Calibrated

HSM

ODOT SPFs with HSM Adjustment

Factors

ODOT SPFs with ODOT Adjustment

Factors

2015 – 2019 Historical Crash Data

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ADJUSTMENT FACTORS

Adjustment Factor ODOT HSM

Inside shoulder width  

Outside shoulder width  

Depressed median width  

Degree of curvature  

Median barrier  

Outside barrier  

Downstream exit lane change  

Upstream entrance lane change  

Lane addition by ramp  

Lane drop by ramp  

Type A/B weaving section  

Posted speed limit 

Left-side ramp  

Proportion curve  

Lane width 

High volume 

Shoulder rumble strips 

Outside clearance 

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ADJUSTMENT FACTORS – MEDIAN WIDTH

Applies to all mediansHSM Data needs:

• Proportion of segment with median barrier

• Inside shoulder width

• Distance from inside shoulder to barrier face

Applies to only depressed mediansODOT Data needs:

• Proportion of segment with median barrier

• Median width

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ADJUSTMENT FACTORS – DEPRESSED MEDIAN WIDTH

𝐴𝐴𝐴𝐴

₃

= 𝑒𝑒

𝑎𝑎(𝑀𝑀𝑀𝑀−60)×𝐷𝐷𝑀𝑀

𝑎𝑎 = regression coefficient MW = median width in feet

DM = indicator for depressed median (1=yes; 0 otherwise)

> 60 feet AF < 1.0

< 60 feet AF > 1.0

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ADJUSTMENT FACTORS – LANE CHANGES

Combines both directions together, combines HSM downstream exit and upstream entrance, and

includes Type B weaving section presence, proportion, and length in one Adjustment

Factor (AF⁷)

AF⁷– Downstream exit lane changeODOT AF⁸ – Upstream entrance lane change

AF⁹ – Lane add by ramp AF¹⁰ – Lane drop by ramp

AF¹¹ – Weaving section

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ADJUSTMENT FACTORS – LANE CHANGES

AF⁹ – Lane add by ramp AF⁸ – Upstream entrance lane change

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ADJUSTMENT FACTORS – EXIT LANE CHANGE

2.0-Mile Study Segment

7,500 AADT 0.1-mile S-C Lane

𝐴𝐴𝐴𝐴

₇

= 𝑒𝑒

𝑎𝑎(𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 𝐷𝐷𝑃𝑃𝐷𝐷𝑃𝑃𝐷𝐷𝑃𝑃𝑃𝑃𝐷𝐷𝑎𝑎𝐷𝐷 𝐿𝐿𝐿𝐿 −0)

𝑃𝑃𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 𝐷𝐷𝑟𝑟𝐷𝐷𝑟𝑟𝐷𝐷𝑟𝑟𝑟𝑟𝑒𝑒𝑎𝑎𝐷𝐷 𝐿𝐿𝐿𝐿 = 𝑆𝑆𝐷𝐷𝑆𝑆𝐷𝐷𝐷𝐷𝑃𝑃𝑃𝑃 𝐿𝐿𝐷𝐷𝑃𝑃𝑆𝑆𝑃𝑃𝐿 𝐷𝐷𝑃𝑃𝑃𝑃𝐿𝑃𝑃𝑃𝑃 0.5 𝐷𝐷𝑃𝑃𝑚𝑚𝐷𝐷𝐷𝐷 𝑃𝑃𝑜𝑜 𝐷𝐷𝑃𝑃𝐷𝐷𝑃𝑃𝐷𝐷𝑃𝑃𝑃𝑃𝐷𝐷𝑎𝑎𝐷𝐷 𝐸𝐸𝐸𝐸𝑃𝑃𝑃𝑃

𝑇𝑇𝑃𝑃𝑃𝑃𝑎𝑎𝑚𝑚 𝑆𝑆𝐷𝐷𝑆𝑆𝐷𝐷𝐷𝐷𝑃𝑃𝑃𝑃 𝐿𝐿𝐷𝐷𝑃𝑃𝑆𝑆𝑃𝑃𝐿 𝑃𝑃𝑃𝑃 𝑀𝑀𝑃𝑃𝑚𝑚𝐷𝐷𝐷𝐷 × (

𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷 𝐸𝐸𝐸𝐸𝐸𝐸𝐷𝐷 𝐴𝐴𝐴𝐴𝐷𝐷𝐴𝐴 1,000

𝐷𝐷𝑃𝑃𝐷𝐷𝑃𝑃𝐷𝐷𝑃𝑃𝑃𝑃𝐷𝐷𝑎𝑎𝐷𝐷 𝐸𝐸𝐸𝐸𝑃𝑃𝑃𝑃 𝐿𝐿𝐷𝐷𝑃𝑃𝑆𝑆𝑃𝑃𝐿 𝑃𝑃𝑃𝑃 𝑀𝑀𝑃𝑃𝑚𝑚𝐷𝐷𝐷𝐷)

𝑃𝑃𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 𝐷𝐷𝑟𝑟𝐷𝐷𝑟𝑟𝐷𝐷𝑟𝑟𝑟𝑟𝑒𝑒𝑎𝑎𝐷𝐷 𝐿𝐿𝐿𝐿 = 15.0

0.3-Mile Study Segment

7,500 AADT 0.1-mile S-C Lane

𝑃𝑃𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 𝐷𝐷𝑟𝑟𝐷𝐷𝑟𝑟𝐷𝐷𝑟𝑟𝑟𝑟𝑒𝑒𝑎𝑎𝐷𝐷 𝐿𝐿𝐿𝐿 = 75.0

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ADJUSTMENT FACTORS – LANE ADDITION/DROP BY RAMP

𝐸𝐸𝑟𝑟𝐸𝐸_{𝐴𝐴𝐴𝐴𝐷𝐷𝑇𝑇} = Entrance ramp AADT

LA = Indicator for the presence of a lane addition by entrance ramp (1 if yes, 0 if otherwise) 𝑎𝑎 = Regression coefficient

𝐴𝐴𝐴𝐴

₉

= 𝑒𝑒

^{𝑎𝑎 𝐸𝐸𝑃𝑃𝐸𝐸}1,000 −0 ×𝐿𝐿𝐴𝐴^{𝐴𝐴𝐴𝐴𝐷𝐷𝐴𝐴}

𝐸𝐸𝐸𝐸𝐸𝐸_{𝐴𝐴𝐴𝐴𝐷𝐷𝑇𝑇} = Exit ramp AADT

LD = Indicator for the presence of a lane drop by exit ramp (1 if yes, 0 if otherwise) 𝑎𝑎 = Regression coefficient

𝐴𝐴𝐴𝐴

₁₀

= 𝑒𝑒

^{𝑎𝑎 𝐸𝐸𝐸𝐸𝐸𝐸}1,000 −0 ×𝐿𝐿𝐷𝐷^{𝐴𝐴𝐴𝐴𝐷𝐷𝐴𝐴}

Lane Addition by Ramp

Lane Drop by Ramp

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ADJUSTMENT FACTORS – WEAVING SEGMENTS

𝐴𝐴𝐴𝐴

₁₁

= 𝑒𝑒

𝑎𝑎(𝑀𝑀𝐷𝐷𝑎𝑎𝑊𝑊𝑃𝑃𝑃𝑃𝑆𝑆 𝑇𝑇𝑇𝑇𝑃𝑃𝐷𝐷 𝑋𝑋)

𝑎𝑎 = regression coefficient

Weaving Type X = Indicator for the presence of applicable weaving section (1=yes; 0 otherwise)

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ADJUSTMENT FACTORS – POSTED SPEED LIMIT

𝐴𝐴𝐴𝐴

₁₂

= 𝑒𝑒

0.012 ×(𝑃𝑃𝑆𝑆𝐿𝐿 −𝐵𝐵𝐿𝐿)

𝑃𝑃𝑆𝑆𝐿𝐿 = Posted speed limit in mph

BL = Baseline for rural (70 mph) or urban (65 mph) segment

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ADJUSTMENT FACTORS

Lane Width

ODOT roadways all had 12-foot lanes, unable to determine effect

High Volume

ODOT data not readily available

Shoulder Rumble Strips ODOT data not readily available

Clear Zone Width

Offsets to Barrier

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EVALUATION BETWEEN MODELS

Uncalibrated HSM

Ohio- Calibrated

HSM

Factors

Mean Absolute Deviation (MAD) Root-Mean-Square Error (RMSE) Mean Absolute Prediction Error (MAPE)

CURE Plots

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EVALUATION BETWEEN MODELS

Uncalibrated HSM

Ohio- Calibrated

HSM

Factors

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EVALUATION BETWEEN MODELS

Ohio- Calibrated

HSM

Factors

• Tends to underpredict crashes

• Applies more variables; more sensitive to geometric factors

• Provides a better fit for the speed-change lanes

• Tends to overpredict crashes

• Applies fewer variables; easier to implement

• Provides better fits by severity level and for the basic freeway segments

(45)

DATA NEEDS

^{Data Input}^{Area Type} ^ODOT^ ^HSM^

Length of freeway segment or s-c lane  

Number of directional through lanes  

Lane width 

Inside and outside shoulder widths  

Median width  

Clear zone width 

Presence of rumble strips 

Freeway Segment AADT  

Proportion of AADT during high-volume hours  Distance to upstream entrance/downstream exit  

Side of freeway ramp enters/exits  

Ramp AADT  

Curve length and radius  

Length of median and outside barrier  

Offset to barrier from traveled way 

Posted speed limit 

Type of weaving segment (A or B)  

Presence of Lighting* 

*Needed for Severity Distribution

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NEXT STEPS FOR OHIO

o Update the ODOT-specific HSM analysis tools to reflect new SPFs and new adjustment factors

o Improve the fit of the models through more extensive data collection

o Horizontal curves o Clear zone

o Barrier presence and offset o Volume implications

Factors

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