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Traffic Safety Analysis ‐ From Finding The Problem To Fixing It

Class Notes, References & Supporting Documentation

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Local Technical Assistance Program 309 Dillman Hall Michigan Technological University Houghton, MI 49931-1295 906-487-2102

Module 1: Overview of Your Crash Data

NOTES: MICHIGAN VEHICLE CODE (EXCERPT) 257.622 ‐ Sec. 622. Report of accidents resulting in death, personal injury, or property damage; forms; analysis; use; retention. The driver of a motor vehicle involved in an accident that injures or kills any person, or that damages property to an apparent extent totaling $1,000.00 or more, shall immediately report that accident at the nearest or most convenient police station, or to the nearest or most convenient police officer. The officer receiving the report, or his or her commanding officer, shall immediately forward each report to the director of the department of state police on forms prescribed by the director of the department of state police. The forms shall be completed in full by the investigating officer. The director of the department of state police shall analyze each report relative to the cause of the reported accident and shall prepare information compiled from reports filed under this section for public use. A copy of the report under this section and copies of reports required under section 621 shall be retained for at least 3 years at the local police department, sheriff's department, or local state police post making the report.00 of 1949 UD‐10 forms: The UD‐10 Traffic Crash Report is used in the State of Michigan by police agencies to report all traffic related motor vehicle crashes. The form is also used to record any crash involving a snowmobile or off‐road vehicle (ORV), whether traffic or non‐traffic related. Crash Severity Scale National Safety Council KABCO Scale:

K – Fatal Injury A – Incapacitating Injury B– Non‐incapacitating Injury C– Possible Injury O– No Injury

MALI – Michigan Accident Locations Index: Linear referencing system that is a table of all public roads by county and city that lists their start point and mileage to intersections including the name of the cross street. Crash Location: In Michigan all crash reports are sent to the Michigan State Police Criminal Justice Center in Lansing, MI where they are checked and physically located using the On‐From locating system which references crash locations from the nearest known intersection. Example “In the City of Houghton, on Sheldon Ave (US 41), 120 feet west from the intersection of Dodge Street”. About 70 to 80 percent of all crashes auto locate using this data, the remaining crashes need to reviewed by a technician.




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Michigan Crash Types: The Michigan State Police have 10 basic crash types. MDOT expands these 10 basic crash types to 27 crash types based on circumstances surrounding the crash. Example MSP has “right angle” as a crash type, MDOT expands this to “angle straight”, “angle drive” and “angle turn”. Crash Type Determination: “Crash Type is based on the intended direction of travel, regardless of point(s) of impact or direction vehicles ultimately face after crash. Damage area on the vehicle alone does not determine crash type. “ 2006 UD‐10 manual p.17 FOIA of Crash Data: 23 U.S.C. § 409 (“Section 409”) was enacted to address this concern. This law expressly forbids the discovery or admission into evidence of reports, data, or other information compiled or collected for activities required pursuant to several Federal highway safety programs (Sections 130, and 152 (now 148)), or for the purpose of developing any high‐way safety construction improvement project, which may be implemented utilizing federal aid highway funds, in tort litigation arising from occurrences at the locations addressed in such documents or data. In 2003, the U.S. Supreme Court upheld the Constitutionality of Section 409, indicating that it “protects all reports, surveys, schedules, lists, or data actually compiled or collected for § 152 purposes” (emphasis on original). Some States consider information covered by Section 409 as an exemption to its public disclosure laws, but courts may not agree with this interpretation.



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RESOURCES

• Many of the supporting documents that don’t have web sites but are used in this class are present at: http://michiganltap.org/workshop/materials/2008Safety/index.html

• Examples of crash report forms from around the country including Michigan’s UD‐10 form. http://www.actar.org/reports.html

• Michigan UD‐10 Manual & Report http://www.michigan.gov/documents/UD‐10_Manual_2004_91577_7.pdf

• Michigan Crash Facts: Annual Crash Data Report http://www.michigantrafficcrashfacts.org/index.htm

• First Edition of the HSM ‐ NCHRP project #17‐36 http// www.highwaysafetymanual.org



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Module 2 Network Screening Methods NOTES: Network Screening: Screening is sometimes referred to as “surveillance”. It is used to find high crash areas of the road network. There are many screening methods available, each with its own benefits and detractions. Dark Spotting: Visually finding high crash locations by plotting them on a map.

Positive o Easy to use and understand o Simple method that can be done quickly with GIS software

Negative o Difficult with large number of crashes o Can be subjective, there are no standards that identify “what too many crashes look like” o Some colors “grab the eye” others “hide” o Spacing and overlap may cause the same number of crashes may appear ‘worse” based on spacing o Zoom Level – May appear better or worse at different magnifications

Crash Frequency: Used at intersections primarily. Crash density is used on segment locations. Is actually a crash “rate” based on time.

Crash Frequency =

= Yearly crashes

Positive

o Easy to use and understand Negative

o Bias for high volume intersections ‐ more traffic = more crashes o Doesn’t take in to account exposure o Need to stratify network to account for bias, Example – High volume, med volume, low volume



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Crash Density: Used on road segments away from intersections. Similar to crash frequency for intersections except it takes into account the length of the road segment.

Crash density =

= Yearly crashes per mile

Positive

o Easy to use and understand Negative

o Segmentation can be an issue, either dividing “dense locations” or having small segments with a few crashes that look inordinately bad due to the small segment size

o Bias for high volume location ‐ more traffic = more crashes o Doesn’t take in to account exposure o Need to stratify network to account for bias, Example – High volume, med volume, low volume

Crash Rate: Can be used for intersections or can be used for road segments if segment length is accounted for.

Crash Rate Intersection = , ,

= Crashes per million entering vehicles (MEV)

Crash Rate Segment = , ,

= Crashes per mile per million entering vehicles

Positive

o Easy to use and understand o Accounts for exposure using traffic volumes

Negative

o Bias for low volume locations with a few crashes o Assumes relations between crashes and volume are linear, which is generally considered not true o Need full volume information to screen network



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Equivalent Property Damage Only Method: Used for intersections. Equates the cost of crashes of different severity to a dollar amount that the crash costs society.

EPDO = ∑ ∑

∑

∑ = Number of EDPO Crashes

Modified EPDO = ∑ ∑ &

∑ = Number of EDPO Crash

Positive

o Easy to use and understand o Takes in account severity

Negative

o Bias for low volume locations with a small number of sever crashes o Does not account for exposure

Weighting Methods: Used for intersections. Equates negative or positive point scores to specific features of crashes based on conditions. Creates a unified tally based on the total points.

Positive

o Can identify very specific details of a crash location o Can account for severity

Negative

o No guidance for assigning weights o Does not account for exposure



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RESOURCES:

• FHWA Report on Crash Costs: Crash Cost Estimates by Maximum Police‐Reported Injury Severity Within Selected Crash Geometries http://www.tfhrc.gov/safety/pubs/05051/05051.pdf

• National Safety Council Estimates of Crash Costs http://www.nsc.org/lrs/statinfo/estcost.htm

• SEMCOG Safety Manual ‐ Information on screening methods http://www.semcog.org/WorkArea/showcontent.aspx?id=3496

• Paper from Iowa DOT on crash Methodologies http://www.dot.state.ia.us/crashanalysis/pdfs/sicl_methodologies.pdf

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RESOURCES

• Michigan UD‐10 Manual & Report http://www.michigan.gov/documents/UD‐10_Manual_2004_91577_7.pdf

• First Edition of the HSM ‐ NCHRP project #17‐36 Chapter 4 Network Screening http// www.highwaysafetymanual.org



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Module 4: Site Level Collision Analysis

NOTES: Crash Diagrams: The goal in creating a crash diagram is to uncover any patterns in the data which may point to causal factors that can be corrected or mitigated. Crash Data History: Typically use 3 or more years of data for analysis. Too short of a period makes analysis more subject to anomalous results from one time or rare events. Too long of a time may show results of changes in traffic volume, or physical changes at the intersection. Site Visits: Site visits are necessary to give context to the data and to give clues to causal factors. The value of observing a location first hand can not be over stated. Site inspection check lists are included in Appendix B of NCHRP 457 RESOURCES

• NCHRP Report 457: Evaluating Intersection Improvements Site inspection check list ‐ Appendix B http://onlinepubs.trb.org/Onlinepubs/nchrp/esg/esg.pdf

• First Edition of the HSM ‐ NCHRP project #17‐36 Chapter 3 Fundamentals Chapter 5 Diagnosis http// www.highwaysafetymanual.org



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Module 5: Site Level Collision Analysis

NOTES: Geometric Analysis: This type of analysis looks for problems or deficiencies associated with the geometric design of the facility. Deficiencies such as poor sight distance, excessively steep road grade or poor curve radius would be likely to be uncovered in this type of a review. The main resources are the AASHTO Green Book and MDOT’s Local Agency Guidelines for 3R and 4R projects. Condition Diagram This is a basic sketch that may be roughly to scale. An example is shown in the figure below.

• Traffic control / signing / marking

• Number of lanes

• Direction of travel

• Parking location

• Obstructions

• Approach grade

• Medians or islands

• Intersection alignment

Figure 3: Example Condition Diagram



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Operational Analysis: This analysis is targeted at determining if poor operating conditions may be leading to traffic crashes.

• Speed

• Traffic volume

• Delay, density and gap studies

• Signal timing

Human Factors Analysis: This class of analysis attempts to determine how the user perceives or reacts to the road environment and looks for areas that may “trick” or confuse drivers. Generally speaking human factors relate to the following questions:

• Visibility: Can you see “it”?

• Expectancy: Do you expect “it” to be there?

• Consistency: Is “it” the same at other locations?

• Visual workload: Can you understand what “it” is? Aerial Photos: RoadSoft can import SID and SHP files to display alongside the road network. The Michigan Center for Geographic Information (CGI) has a rich source of aerial photos and other shape files in the Michigan Geographic Data Library at: http://www.mcgi.state.mi.us/mgdl Under the “Theme” section of the library there are digital resources covering aerial imagery, census data, geology data, groundwater and surface water data, section lines, and soil types to name a few. Almost all of this data can be directly imported into RoadSoft as a layer.



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RESOURCES

• NCHRP Report 457: Evaluating Intersection Improvement Site inspection check list ‐ Appendix B http://onlinepubs.trb.org/Onlinepubs/nchrp/esg/esg.pdf

• Michigan DOT Local Agency Guidelines for 3R and 4R projects http://www.michigan.gov/documents/MDOT__geometrics_77543_7.pdf

• AASHTO Green Book: Geometric Design of Highways and Streets – For sale at: http://www.bookmarki.com/AASHTO_Green_Book_A_Policy_on_Geometric_Design_p/1560512636.htm http://www.techstreet.com/cgi‐bin/detail?product_id=1183385 https://bookstore.transportation.org/Item_details.aspx?id=109

• NCHRP 336: Road Safety Audits http://onlinepubs.trb.org/Onlinepubs/nchrp/nchrp_syn_336.pdf

• Transportation Research Board: Highway Capacity Manual http://www.trb.org/bookstore/

• Iowa State University: Handbook of Simplified Practices for Traffic Studies http://www.ctre.iastate.edu/PUBS/traffichandbook/

• Manual of Transportation Engineering Studies http://www.ite.org/bookstore/

• NCHRP 600 Human Factors Guidelines for Road Systems, http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_600A.pdf

• Human Factors in Traffic Safety, Second Edition (book) by Robert E., Ph.d. Dewar, Paul L. Olson

• Michigan Geography Data Library http://www.mcgi.state.mi.us/mgdl

• First Edition of the HSM ‐ NCHRP project #17‐36 http// www.highwaysafetymanual.org



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Module 6: Selecting Countermeasures

NOTES: Many factors influence the occurrence of crashes. Several research projects have shown that crashes usually occur as a result of several factors, not usually just one factor. Some factors that may cause crashes include the following:

• Driver Condition o Reflexes o Attentiveness o Experience o Alcohol & drug use o Driver aggressiveness

• Human Factors o Visibility o Expectancy o Consistency o Workload

• Vehicle o Handling characteristics o Maintenance

• Roadway o Geometry o Maintenance o Surface condition

• Environmental o Rain / snow / fog

Steps in identification of deficiencies and selection of countermeasures

1. Detect underlying crash pattern if any 2. Use other site level analysis to identify factor 3. Determine possible causal factors 4. Determine potential remedies 5. Evaluate potential crash reduction for remedies 6. Select most cost effective remedy

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Roadway Environment

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RESOURCES

• MDOT Time Of Return Spreadsheet http://michiganltap.org/workshop/materials/2008Safety/index.html

• NCHRP Synthesis 321: Roadway Safety Tools For Local Agencies http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_syn_321.pdf

• SEMCOG Traffic Safety Manual Table 4‐5 to 4‐10 causes and countermeasures http://www.semcog.org/TranPlan/Safety/assets/Safety_Manual.pdf

• NCHRP 500 Series Manuals – Guides for Reducing Collisions at Specific Locations http://safety.transportation.org/guides.aspx

• FHWA Desktop Reference For Crash Reduction Factors http://www.transportation.org/sites/safetymanagement/docs/Desktop%20Reference%20Complete.pdf

• Minnesota Benefit Cost Analysis For Transportation Projects – Guide http://www.oim.dot.state.mn.us/EASS/BCA‐Guidance‐08‐15‐05v2.pdf



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Michigan DOT Safety Improvements and Crash Reduction Factors

Proposed Improvement Projected Crash Reduction

Horizontal Curve Flattening 30% Reduction: Head-On, Sideswipe, Fixed-Object, Overturn Superelevation Modification 20% Reduction: Head-On, Sideswipe, Fixed-Object, Overturn

Vertical Curve Modification 20% Reduction: Head-On, Sideswipe 10% Reduction: Fixed-Object, Overturn

Construct Center Left-Turn Lane

80% Reduction: Rear-End Left-Turn 50% Reduction: Head-On Left-Turn 20% Reduction: Head-On, Angle, Other* 15% Reduction: Non Left-Turn Rear-End

Construct Right-Turn Lane 65% Reduction: Rear-End Right-Turn 20% Reduction: Non Right-Turn Rear-End, Sideswipe-Same Direction

Intersection Improvements (Realignment, Sight-Distance Improvements, Radii Improvements, Etc.)

30% Reduction: Angle 15% Reduction: Rear-End 10% Reduction: Head-On, Sideswipe, Pedestrian, Bicycle, Left-Turn Related

Install/Upgrade Flashing Traffic Signals 30% Reduction: All Crash Types

Install/Upgrade Pedestrian Signals 30% Reduction: Pedestrian, Bicycle

Install Guardrail 55% Reduction: Fatalities and “A” Injuries

Slope Flattening 15% Reduction: Fixed-Object, Overturn

Widen Shoulders to Standard Width 15% Reduction: All Crash Types

Improve/Upgrade Signing and Pavement Markings at Intersections

30% Reduction: Angle, Rear-End 10% Reduction: Head-On, Pedestrian

Install/Upgrade Signing/Delineation on Horizontal Curves 20% Reduction: Head-On, Sideswipe, Fixed-Object, Overturn

Remove Fixed-Objects From Clear Zone (Trees, Culverts, Etc.) 75% Reduction: Fixed-Object Crashes

Install Centerline Rumble Strips 55% Reduction: Sideswipe-Opposite, Head-On

Install Shoulder Rumble Strips 25% Reduction: Fixed-Object, Overturn

Construct Roundabout 76% Reduction: Fatalities and “A” Injuries 39% Reduction: Minor & PDO Crashes

Construct Sidewalk for Pedestrians 85% Reduction: Pedestrian Crashes Improve Access Management 10% Reduction: Angle, Rear-End Provide All-Way Stop-Control Operation at Intersection 60% Reduction: All Crash Types

* “Other” includes any other crash which might be mitigated by the addition of a center left‐turn lane in the judgment of the crash analyst. The references listed below are the sources recognized by MDOT for obtaining crash reduction factors. If you have a situation that none of these sources can provide a crash reduction factor for, please contact Jim D’Lamater, P.E., MDOT Local Agency Programs Safety Engineer, at 517‐335‐2224, for review and approval to use alternative reference sources. 1). MDOT Safety Programs Unit ‐ Crash Reduction Factors (As recommended by K. Kunde, P.E.); October, 1986. 2). Selection Process for Local Highway Safety Projects ‐ Transportation Research Record 847; 1982. 3). UKTRP – 85‐6, University of Kentucky; March, 1985.



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Module 7: Before and After Studies

NOTES: Regression to the mean: Crashes are psudo‐random events, meaning that they are hard to predict and have a random component to their occurrence. Because of this, for a given site the number of crashes will fluctuate up and down over time, but over the long run, if things are not changed, the value will hover around an average value. When a site is picked for study solely on the fact it has a high incidence of crashes over a short period of time, it will be subject to regression to the mean bias.

Regression To The Mean Bias

Average = 31

Figure 4: In 2004 this site had a high incidence of crashes. Even if nothing was done to change safety, the next year it had 19 fewer crashes.

Confounding Factors: Changes in traffic volume, land use, vehicle or driver population and even weather can confound standard before and after analysis if not corrected for.



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Direct Reporting of Results: Is an easy method of getting some indication of the effectiveness of a treatment is direct reporting of results. This method may not provide conclusive results and offers no method for correcting for regression to the mean or other confounding factors. Direct reporting should not be the basis of creating crash reduction factors or high value, policy guiding studies. It should only be used as a rough gauge of effectiveness if no other methods are possible.

Figure 5: Example of direct reporting of results

Before After Percent2001 to 2003 2004 to 2006 Change

Targeted CrashesCorrectable Crashes 21 11 -48%Other Intersection Crashes 25 21 -16%Total 46 32 -30%

SeverityIncapacitating Injury & Fatal 15 7 -53%Non Incapacitating Injury 8 11 38%Property Damage Only 23 14 -39%Rate (Per MEV)Crash Rate 2.35 1.54 -34%Severe Injury and Fatality Rate 0.77 0.34 -56%ExposureNetwork Wide Crashes (Total) 17,990 18,048 0.3%Traffic Volume @ Location (MEV) 19.57 20.78 6%



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Test For Statistical Significance:

Z1 = Test Statistic fa = # Crashes After fb = # Crashes Before

Example: Before = 15 Crashes After = 12 Crashes = 0.577

Compare Z score to standard normal table to see the level of significance of change.

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Standard Normal Table



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Naïve Before And After Study: This method uses crash counts during a period before and after to determine a change in safety. Usually 3 years of data for each period is used. For the study to be considered correct and valid, the assumption that no other factors have changed during the study period which include changes in (traffic, weather, vehicle driver behavior, etc) must be made. Typically these assumptions are hard to meet, but in most case this method will provide reasonable results.

Naïve Before and After Calculations:

λ = sum of crashes in “after” period K = sum of crashed in “before” period rd = duration of after period / duration of before π = rd (K ) = estimate of untreated “after” period πvar = (rd )

2(K ) = variance of untreated after period σ= π‐λ = Reduction in crashes due to treatment σ var = (π‐λ)

0.5 = standard deviation of reduction

Index of effectiveness

Standard Deviation of effectiveness Before and After Study with Comparison Group: This method is useful for large group comparisons of many sites. The method uses a comparison group of untreated locations which are similar to the locations of study as a proxy to measuring the change in safety that develops as a result of confounding factors. The method gives good results but will not correct for regression to the mean.

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Emperical‐Bayes Method: This method corrects for regression to the mean bias and changes in volume. It can correct for other factors that may change during the study as well. However this method requires significant data and skill as a statistician, and may be difficult to explain and tedious to complete. This method should be used for high value studies where significant confidence in the results is required. This method is beyond the scope of this class.



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RESOURCES

• Naïve Before And After Worksheet http://michiganltap.org/workshop/materials/2008Safety/index.html

• Before and After With A Comparison Group ‐ Worksheet http://ca.geocities.com/[email protected]/Pubs/cgzipexcell.ZIP

• Ezra Heur’s web site http://www.roadsafetyresearch.com/

• Observational Before‐After Studies in Road Safety by Dr. Ezra Hauer Instructions on Empirical Bayes method http://www.amazon.com/Observational‐Before‐After‐Studies‐Road‐Safety/dp/0080430538

• First Edition of the HSM ‐ NCHRP project #17‐36 Chapter 9 Evaluation of Effectiveness – Empirical‐Bayes Method Comparison Group Studies http// www.highwaysafetymanual.org

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