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CE 552 Week 13

Identifying problems and solutions – The safety audit (materials, chapter 9)

Responses/countermeasures/Crash reduction factors/AMFs (materials, chapter 10)

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• Improve safety• Interdisciplinary teams• Existing or new

roadway– Planning– Construction– Design

• ID problem elements• ID opportunities

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• Team quals: • Crash investigation• Traffic engineering• Road design• Enforcement• Maintenance• EMS• Human factors

• Background info• Plans, volumes,

crash data5

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Development of countermeasures

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Principles of countermeasure development

• The process of countermeasure development should aim to– Determine a Range of measures likely to influence the

dominant crash types and road features– Select countermeasures based on professional

judgment and experience that can expected to reduce the number or severity of dominant crashes

From Ogden, Chapter 7

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Principles of countermeasure development (continued)

– Check to see if adopted countermeasures have any undesirable consequences in

• Safety terms– e. g. lead to an increase in number or severity of another

crash type• Traffic efficiency• Environmental terms

– Be cost-effective• Maximize the benefits from HRL program

– Be efficient• Produce benefits which outweigh costs

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Principles of countermeasure development (continued)

• A safe road is one that recognizes the realities and limitations of human decision making

• The management of road safety must ensure the road environment not place demands upon the driver that are beyond the driver’s ability to manage, or which are outside normal road user expectations

• What are the characteristics of a safe road?

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Characteristics of a safe road

• A safe road is defined as one which is designed and managed so that it:– Warns the driver of unusual features– Informs the driver of conditions to be encountered– Guide the driver through unusual sections– Control the driver’s passage through conflict points and

road links– Forgives a driver’s inappropriate behavior

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Intersections • The main design principles for intersections are

– Minimize the number of conflict points and thus the opportunity for crashes

• T-intersections and roundabouts have fewer conflict points than 4-way intersections

– Give precedence to major movements through• Alignment• Delineation• Traffic control

– Separate conflicts in space or time

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Intersections (continued)– Control the angle of conflict

• Crossing streams of traffic should intersect at a right angle or close to it

• Merging streams should intersect at small angles to ensure low relative speed

– Define and minimize conflict areas– Define vehicle paths– Ensure adequate sight distances– Control approach speed using

• Alignment• Lane width• Traffic control• Speed limits

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Intersections (continued)– Provide clear indications of right-of-way requirements– Minimize roadside hazards– Provide access to use intersection for

• Vehicular traffic– Special provisions for heavy vehicle and public transportation

vehicles

• Non-vehicular traffic– Pedestrians and other vulnerable road users

– Simplify the driving task– Minimize road user delayRoundabouts usually include to some degree allof the above principles

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Mid-block locations• The principles for design and operation of non-

intersection locations include– Consistent standards of horizontal and vertical

alignments– Develop roadway cross sections to suit road function and

traffic volumes– Delineate roadway and vehicle paths– Standards of access control from abutting land use– Ensure that roadside environment is clear or forgiving

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Mid-block locations (continued)• Overlaying all of the above principles is a vital need to

consider particular needs of all road user groups• Careful consideration of these needs will ensure the

quality of final treatment– Pedestrians have special needs that should be separately

considered when investigating safety problems and developing countermeasures

– Special requirements of heavy vehicles• Negotiating low-radius turn• Traveling through horizontal curves with adverse super-elevation

– Other user groups needs

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Countermeasure selection

• Matching solutions to problems– The key to selection of countermeasures is to

concentrate on the particular crash types which identified in diagnosis phase

– The final choice will be based upon judgment and experience

– Utilizing countermeasures which have been successful in similar situations elsewhere

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Criteria for countermeasure development

• There are number of criteria for countermeasure selection– Technical feasibility

• Can countermeasure provide an answer? • Does it have technical basis for success?

– Economic efficiency• Is countermeasure likely to be cost effective?• Will it produce benefits to outweigh its costs?

– Affordability• Can it be accommodated through program budget?• Should a cheaper solution be adopted?

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Criteria for countermeasure development (continued)

– Acceptability • Does the countermeasure target the problem?• Will it easily understandable by community?

– Practicality• Is there likely to be a problem of non-compliance?• Can the measure work without unreasonable enforcement

effort?

– Political and institutional acceptability• Is the countermeasure likely to attract political support?• Will that be supported by the organization responsible for its

installation and on-going management?

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Criteria for countermeasure development (continued)

– Legal compatibility• Is the countermeasure a legal device?• Will users breaking any law by using it in the way intended?

– Compatibility• Is the countermeasures compatible with other strategies which

have been applied in similar situations?• It can be seen that adapting countermeasures to particular

problem is a complex process. • Development of countermeasures requires understandable

technical and institutional framework to provide the guideline principles and motivation for action

From Garber …

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From: Khisty

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1990 NHTSA/FHWA effective safety measures

What’s new since then that you know about?37

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Crash Reduction Factors

Aka Accident Modification Factors or Functions

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Do you know another way to do this?

See: combining crash reduction factors 57

Access Control

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Alignment

• Curved sections have 1.5-4.0 times the crash rate of tangent sections

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Cross sections

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Intersections

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Pedestrians

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Crash Reduction Factors for Countermeasures

Issue Briefs and Desktop Reference

Maurice Masliah, Ph.D. Senior Researcher iTRANS Consulting

(905) 882-4100 ext 5295 mmasliah@itransconsulting.com

Prepared by iTRANS Consulting

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April 2004

September 2007

Updated Issue BriefsTraffic Signals Intersections

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New Issue BriefsRoadway Departure Pedestrian

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New Product: Desktop Reference

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What is a CRF?

• CCrash RReduction FFactor

• Percentage crash reduction that might be expected after implementing a countermeasure– CRF of 25 means a 25% reduction in crashes

– CRF of -20 means a 20% increase in crashes

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Issue Briefs: Purpose

• One CRF for each countermeasure

• The “best” available CRF (accuracy and precision checks)

• Additional information relevant to the CRF(traffic control, area type, etc.)

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Desktop Reference: Purpose

• Multiple CRFs for the same countermeasure (where available)

• CRF functions

• Additional information such as study type, number of observations, CRF range (high - low)

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Product PurposeIowa DOT

Tom WelchState Transportation Safety Engineer

Iowa Dept of Transportation

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Roadway Departure Issue Brief

• Purpose

• CRFs

• How to Use

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Roadway Departure CRF Example

• CRF = 38(10)17

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CRF Example

Remove or relocate fixed objects outside the clear zone

38CRF

Expected crashes without treatment

Expected crashes with treatment

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CRF Example

• CRF = 38(10)17

Bold type means a rigorous methodology was used to estimate the CRF, and the standard error is relatively small

(10 ) is the standard error for this CRF

17 is the reference number* * details at the end of the Issue Brief

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CRF Confidence Interval

• Estimation of confidence interval [38(10)17]

2 standard errors from the CRF

18% (38 - 2 X 10) and 58% (38 + 2 X 10)

• Expected safety effect of the countermeasure: between 18% and 58%

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Example ApplicationDouglas County

Keary B. LordAssistant Director/

Traffic Operations Division Manager

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Accuracy

• Accuracy is the proximity of the estimate of the CRF to the true value

• Two types of bias affect accuracy:

1. Regression to the mean bias

2. Traffic volume bias

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Precision

• Precision is the degree to which repeated estimates of the CRF are similar to each other

• Measure of precision: standard error of CRF

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Reliability

• Reliability is defined here as the combination of precision and accuracy

• CRFs that pass the reliability test are shown in bold font

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Reliability Selection Criteria and Thresholds

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Number of CRFs and Bold CRFs

Total CRFs

Bold CRFs

% Bold CRFs

Intersection 452 63 14% Roadway Departure 504 28 6% Pedestrian 32 2 6%

** Very few published CRFs Very few published CRFs are considered reliable!are considered reliable!

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CRF Proper Use and Limitations Colorado DOT

TBD – Bryan Allery/Jake Kononov

Colorado Dept of Transportation

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Highway Safety Improvement Program (HSIP)

• Planning • Implementation • Evaluation

(3) A comparison of accident numbers, rates, and severity observed after the implementation of a highway safety improvement project with the accident numbers, rates, and severity expected if the improvement had not been made.

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HSIP & CRFs

• Evaluation legislation specifies the information needed to produce CRFs

• Our review of HSIPs has not found any states which tie their evaluation work with CRF generation

• Potential to link tasks currently being conducted with improving knowledge of effectiveness of countermeasures

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Strategic Highway Safety Plan (SHSP)

• CRFs should be part of the process of selecting countermeasures

• Intersections as an emphasis area within a SHSP

• Task team identifies left-turn crashes at intersections as an overrepresented crash type

• What are appropriate countermeasures?

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Strategic Highway Safety Plan (SHSP)

• Review of Intersection Issue Brief for left-turn crash CRFs– Provide protected left turn phase 16(2)19

– Install left-turn lane (physical channelization) 249

– Install cameras to detect red-light running 45(6)27

• Identify intersection locations and conduct a cost-benefit analysis

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Future Direction – What’s Next?

• Accident Modification Factors and Functions (rather than CRFs)

• Increase in number of studies that evaluate safety effects of countermeasures

• More stringent criteria in adoption of AMFs

• New tools

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What is an AMF?

• Multiplier representing the expected change in crashes

– CRF of 25 equals an AMF of 0.75

– CRF of -20 equals an AMF of 1.20

• Accident Modification Factor

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Accident Modification Functions

• Today, CRFs / AMFs for most countermeasures are noted as constants

• However, CRFs / AMFs are really functions of environmental variables:

– Traffic volume

– Traffic mix (trucks, pedestrians, bicyclists)

– Road geometry

– Operational conditions

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More Stringent Inclusion Criteria

• Highway Safety Manual inclusion process:

– Accuracy and precision of AMFs

– Standard error of 0.1 or less indicates AMF that is sufficiently accurate, precise, and stable

– Companion AMFs with standard errors of 0.2 - 0.3 are also included

• indicating the potential safety effects of the treatment on other facilities, or other crash types and severities

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Future Direction

• Periodical Update of Issue Briefs and Desktop Reference

• Search new information: – current research projects: http://rip.trb.org/– government-funded documents published:

http://www.ntis.gov/search/index.asp?– bibliographic database

http://ntlsearch.bts.gov/tris/index.do

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New Tools and New AMFs

• Low Cost Safety Improvements Pooled Funds Study

• Highway Safety Manual (HSM)

• SafetyAnalyst

• Interactive Highway Safety Design Model (IHSDM)

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CREDITS

The Technical Working Group (TWG)FHWAProject Manager

Clayton Chen

FHWA Marcee AllenNick ArtimovichMark DoctorKenneth EpsteinRay KrammesJim GrowneyDean Larsen

Matt LupesMary McDonoughDavid MorenaFred RanckTamara RedmonEd RiceGabe Rousseau

Iowa DOT Tom Welch

ITE Ed Stollof

iTRANS Geni BaharMaurice MasliahRhys WolffPeter ParkNesta Morris

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Many more like this in the reference 102