Incorporating Safety into the Highway Design Process.
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Incorporating Safety into the Incorporating Safety into the Highway Design ProcessHighway Design Process
What is Meant by “Safety”?What is Meant by “Safety”?• Is This Road Safe?
– Is a “Yes” or “No” answer sufficient?
– Would your answer change if you were told...• The road averages 1 crash in 10 years? or...• The road averages 100 crashes in 10 years?
Kinds of SafetyKinds of Safety
• Nominal Safety– A road that conforms to the agency’s policy,
guidelines, and warrants is “nominally” safe
– A road either is, or is not, nominally safe
• Substantive Safety– The performance of a roadway, as defined by
its “expected” crash frequency (i.e., long run average)
– Substantive safety is a continuous variable
– Useful to compare one site with “typical” site
Safety-Conscious DesignSafety-Conscious Design
• AASHTO Guidance– “Consistent adherence to minimum [design
criteria] values is not advisable”
– “Minimum design criteria may not ensure adequate levels of safety in all situations”
– “The challenge to the designer is to achieve the highest level of safety within the physical and financial constraints of a project”
• Highway Safety Design and Operations Guide, 1997
Highway CrashesHighway Crashes• Contributing Factors
– Driver • Age, gender, skill, fatigue level, alcohol, etc.
– Vehicle • Type, age, maintenance, etc.
– Environment • Light conditions, weather, precipitation, fog, etc.
– Roadway• Geometric design, traffic control, etc.
• Focus of current research– Geometric design of the roadway
Quantifying SafetyQuantifying Safety
• Safety Prediction Model– C = base crash rate × volume × length × AMF
• Accident Modification Factor (AMF)– AMF used to estimate change in crashes due
to a change in geometry (AMF = Cwith/Cwithout)
– Example: • AMFadd bay = 0.70
• Cno bay = 10 crash/yr
• Cwith bay = Cno bay × AMFadd bay = 7 crashes/yr
– Crash reduction factor (CRF) = 1 - AMF
Crash DataCrash Data• Existing Crash Databases
– Texas Department of Public Safety (DPS)– Local databases
• Severity Scale– K: Fatal– A: Incapacitating injury– B: Non-incapacitating injury– C: Possible injury– PDO: property damage only
• Reporting Threshold – $1000, informally varies among agencies
Research focus
Crash Data VariabilityCrash Data Variability• Examination of Crash History
– Annual crash counts: 2, 3, 1, 1, 7, 5, 2...
– Count in any one year is effectively random
– Variability year to year is LARGE
– So large that...• It is very difficult to determine if the change in count
from year to year is due to a change in geometry, traffic volume, or traffic control device
• It can frustrate efforts to reduce crashes (a change was made but crashes increased)
• It can fool us into thinking a change that we made significantly reduced crashes (when it really did not)
Crash Data VariabilityCrash Data Variability
• Questions– What is the true mean crash frequency at
this site?
– Is a 3-year average reliable?
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Each data point represents 1 year of crash data at one site
Crash Data VariabilityCrash Data Variability• Observations
– The average of 3 years (= 6 crashes)...• 2.0 crashes/yr• 0.7 to 4.3 crashes/yr (± 115%)
– The average of 35 years (= 100 crashes)…• 2.8 crashes/yr • 2.2 to 3.3 (± 20%)
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Running Average
Upper Limit (95% confidence interval)
Lower Limit
– One site rarely has enough crashes to yield an average with a precision of ± 20%
Influence of DesignInfluence of Design• Question
– 15 intersections have left-turn bays added
– Research shows bays reduce crashes by 20%
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Before Bay After Bay
Site 4
– What crash frequency do you expect for site 4 after the bay is installed?
Each data point represents 1 year of crash data
Average = 10
Influence of DesignInfluence of Design• Observations
– Random variation makes trend difficult to see
– Most sites show crash reduction
– Site 4, and a few other sites, had more crashes
– This does not mean bay won’t be effective in long run 0
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Before Bay After Bay
Site 4
Site 4
Influence of DesignInfluence of Design• Observations
– Distribution of crash change for sites with average of 10 crashes/yr and 20% reduction
– When reduction is small, random variation will let crash frequency increase at some sites in the year after
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Change in Annual Crash Frequency
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32% of sites experience an increase in crashes in the year after treatment due to random variation
Overcoming VariabilityOvercoming Variability• Large variability makes it difficult to
observe a change in crash frequency due to change in geometry at one site
• Large variability in crash data may frustrate attempts to confirm expected change
• Large databases needed to overcome large variability in crash data
• Statistics must be used to accurately quantify effect
Background ResearchBackground Research• National Research Sources
– Safety design guidelines• NCHRP Report 500: Guidelines for
Implementing the AASHTO Strategic Highway Safety Plan
– Vol. 5: Unsignalized intersections– Vol. 7: Horizontal curves– Vol. 8: Utility poles– Vol. 12: Signalized intersections– Vol. 13: Heavy trucks
• Volumes can be found at:
http://safety.transportation.org/guides.aspx
Background ResearchBackground Research
• National Research Sources– Safety evaluation tools
• Interactive Highway Safety Design Model
• Safety Analyst (forthcoming)
• Highway Safety Manual (forthcoming)
• Prediction of the Expected Safety Performance of Rural Two-Lane Highways
FHWA
FHWAFHWA
NCHRP
Background ResearchBackground Research• TxDOT Project 0-4703
– “Incorporating Safety into the Highway Design Process”
– Project Director: • Elizabeth Hilton
– Main products:• Roadway Safety Design Synthesis
(Report 0-4703-P1)• Interim Roadway Safety Design
Workbook (Report 0-4703-P4)
Available at: tcd.tamu.edu, click on “Products”
Facility TypesFacility Types• IHSDM
– Two lane highways
• Highway Safety Manual– Two lane highways
(& intersections)– Rural multilane
highways (& intersections)
– Urban streets (& intersections)
• TxDOT 0-4703– Freeways– Rural highways
• Multilane rural• Two lane rural
– Urban streets– Freeway ramps– Urban intersections– Rural intersections
Safety Prediction ProceduresSafety Prediction Procedures• Overview
– Six steps to procedure
– Evaluate a specific roadway segment or intersection (i.e., facility component)
– Same basic technique for all methods (IHSDM, HSM, TxDOT 4703)
• Output– Estimate of crash frequency for segment or
intersection
Step 1Step 1
• Identify Roadway Section– Define limits of roadway section of interest
• Limits of design project
• Portion of highway with safety issue or concern
– May include one or more components
Step 2Step 2
• Divide Section into Components– Analysis based on facility components
• One intersection or
• One interchange ramp or
• One roadway segment
– Each component analyzed individually in Steps 3 and 4
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Homogeneous SegmentHomogeneous Segment
• Definition– A homogeneous segment has the same
basic character for its full length• Lane width
• Shoulder width
• Number of lanes
• Curvature
• Grade
• Horizontal clearance
Step 3Step 3• Gather Data for Subject Component
– Data may include• Roadway geometry (lane width, etc.)
• Traffic (ADT, truck percentage, etc.)
• Traffic control devices (stop sign, signal)
– What data do I need?• It depends on the component…
Step 4Step 4
• Compute Expected Crash Frequency– Use safety prediction model
• Model Components– Base model
– Accident modification factors
Volume Lane Width
Expected Crash Frequency
Base ModelBase Model• Relationship
– Cb = base crash rate × volume × length
– Injury (plus fatal) crash frequency
• Calibration– Analyst can adjust crash rate to local
conditions
• Application– Crash frequency for “typical” segment
– Typical: 12 ft lanes, 8 ft outside shoulder, etc.
Accident Modification FactorsAccident Modification Factors
• Definition– Change in crash frequency for a specific
change in geometry
– Adapts base model to non-base conditions
– One AMF per design element (e.g., lane width)
• Example: Two-lane highway
– Base condition: 12 ft lanes
– Roadway has 10 ft lanes
– AMF = 1.12
Steps 5 & 6Steps 5 & 6
• Repeat Steps 3 and 4 for Each Component• Add Results for Roadway Section
– Add crash estimates for all components
– Sum represents the expected crash frequency for the roadway section
• If there are multiple alternatives, repeat Steps 1 through 6 for each alternative
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