reliability guide for the hcm concepts...

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SHRP 2 Project L08: Incorporation of Travel Time Reliability into the Highway

Capacity Manual

July 2012

Reliability Guide for the HCM‐ Concepts & Content

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Research ObjectivesResearch Objectives

(1) Determine how non-recurrent congestion impacts can be incorporated into HCM procedures

(2) Develop methodologies to predict travel time reliability on freeway facilities, urban streets, and freeway-arterial corridors

(3) Prepare draft material (Reliability Guide) for possible future incorporation into the HCM

The objectives of Project L08 are to:

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Anticipated ProductsAnticipated Products

• Computational procedures for assessing travel time reliability– Freeway facilities

– Urban streets

• Draft guidebook for possible inclusion in the HCM

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SHRP 2 Project L08: Incorporation of Travel Time Reliability into the Highway

Capacity Manual

Introduction to Reliability

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Travel Time Reliability: Basic ConceptsTravel Time Reliability: Basic Concepts

• Measured by how travel time of a trip varies from one time period to another

• In other words, reliability is measured as the variability of travel times– “How long will my trip take today compared to the same trip at

the same time on any average day?”

– … this implies …

– Travelers have the ability to predict travel time for a trip and to arrive at destination within an “on-time window”

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Definitions and ConceptsDefinitions and Concepts

StudyPeriod

Study Section

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68 66 60 67 63 39 64 64

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66 65 38 13 11 37 70 70

68 63 62 40 18 38 67 67

63 63 62 68 40 37 68 68

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65 70 64 63 67 64 64 6415:00

18:00

Each cell is one analysis period of an analysis segment.

TemporalDimension

SpatialDimension

Reliability Reporting Period

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What Causes Unreliable Travel?What Causes Unreliable Travel?

n = Source of Congestion

Base Delay(“Recurring” or “Bottleneck”)

PhysicalCapacity

…interacts with… DemandVolume4

Event‐RelatedDelay

TotalCongestion

Daily/SeasonalVariation

SpecialEvents

Planned

…determine…

Emergencies2 31

…lowers capacityand changes demand…

Traffic ControlDevices

Roadway Events

Weather

Incidents

WorkZones

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6

7

…can cause…

…can cause…

…can cause…

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What Features Should Reliability Performance Measures Have?What Features Should Reliability Performance Measures Have?

• Related to measuring variability of travel times• Can be communicated to wide variety of

audiences• Useful in specific types of analyses that use HCM

analyses as their basis• Capable of discerning trends, i.e., sensitive to

changes in the underlying sources of unreliable travel

Many potential measures have these features – can we limit?

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Performance Measuresare Defined by the Travel TimePerformance Measuresare Defined by the Travel Time

0

50

100

150

200

250

300

350

400

4.5 9.5 14.5 19.5 24.5 29.5

Travel Time (in Minutes)

Number of Trips (in Thousands)Free Flow

Mean 95th

Percentile99th

Percentile

Misery Time

Standard Deviation

BufferTime

PlanningTime

Failure Measure

Semi-Standard Deviation

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Recommended Performance MeasuresRecommended Performance Measures

Reliability Performance Measure Definition

Core Measures

Planning Time Index 95th percentile Travel Time Index (95th

percentile travel time divided by the free flow

travel time)

80th Percentile Travel

Time Index

80th percentile travel time divided by the free

flow travel time

Semi‐Standard Deviation One‐sided standard deviation, with reference point at free flow rather than mean

Failure/On‐Time

Measures

Percent of trips with space mean speed less than

50 mph; 45 mph; and 30 mph

Supplemental Measures

Standard Deviation Usual statistical definition

Misery Index (Mod) The average of the highest five percent of travel

times divided by the free flow travel time

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Use Cases for Travel Time ReliabilityUse Cases for Travel Time Reliability

1. Assess existing reliability of facilities. 2. Predict future reliability of facilities.3. Identify options to improve reliability.4. Estimate benefits of improvements.5. Prioritize reliability improvement options.6. Set local standards for acceptable reliability. 7. Improve demand modeling.

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Use Cases for SHRP2-L08/HCMUse Cases for SHRP2-L08/HCM

1. Assess existing reliability of facilities. √2. Predict future reliability of facilities. √3. Identify options to improve reliability.4. Estimate benefits of improvements. √5. Prioritize reliability improvement options.6. Set local standards for acceptable reliability. √7. Improve demand modeling.

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Integrating Safety Into the Project Development Process

James A. Bonneson

Kittelson & Associates

July 2012

Highway Safety & Design

• What is Safety?

• Safety in the Project Development Process

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What is Safety?

• Is This Road Safe?

What is Safety?• Is This Road Safe?

– There are many deficient design elements that make it risky, but is that the same as safe?

• Safety is most clearly defined in terms of crash frequency and severity

• Crash frequency combines risk and exposure (e.g., AADT)

– Is a Yes or No answer sufficient?

• There are degrees of safety

• Estimating the expected crash frequency is a more reliable indicator (as opposed to the previous year’s crash count)

• A road with 14.9 cr/yr is safer than one with 15.2 cr/yr

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Kinds of Safety• Substantive Safety

– Expected crash frequency (i.e., long run average number of crashes per year)

– Substantive safety is a continuous variable

– Useful for comparing alternatives

• 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

Kinds of Safety

• Safety Comparison (NCHRP Report 480)

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Quantifying Safety• Highway Safety Manual

• Safety Prediction Model

– C = base crash rate × volume × length × CMF

• Crash Modification Factor (CMF)

– CMF used to estimate change in crashes due to a change in geometry (CMF = Cwith/Cwithout)

– Example:

• CMFadd bay = 0.70

• Cno bay = 10 crash/yr

• Cwith bay = Cno bay × CMFadd bay = 7 crash/yr

Highway Crashes• Safety Indicators

– Fatal crashes/100 mvm 1.02 1.09 +7%

– Fatalities/100 mvm 1.11 1.17 +5%

– Ped. fatalities/100,000 pop. 1.38 0.84 ‐34%

– 56 fatalities/yr on Alaska highways

• Data for Year 2010

U.S. Alaska Diff.

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Highway Crashes• Contributing Factors

– Driver • Age, gender, skill, fatigue level, alcohol, etc.

– Vehicle • Type, age, maintenance, etc.

– Roadway• Geometric design, traffic control devices, lighting

– Environment • Day/night, precipitation, fog, temperature, etc.

• HSM Focus– Roadway

Highway CrashesROADWAY and ENVIRONMENT

HSM Chapter 3

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Safety in theProject Development Process

• Geometric Design Impacts

– Safety

– Environment

– Traffic operations

– Right‐of‐way

– Construction costs

• Goal

– Find best design that offers minimal total impact

– Cost‐benefit analysis is one way to evaluate

Project Development Process

• Stages– Planning and programming

– Alternative identification and evaluation

– Preliminary design

– Final design

• Where to Evaluate Safety?– All four stages

• How to Incorporate Safety?– Adhere to design criteria and guidelines

– Conduct explicit safety evaluation, as appropriate

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Alternatives Evaluation Stage• Elements

– Traffic analysis

• Evaluate crash data, identify existing patterns

– Preliminary alternatives

• Develop design schematic– Adhere to design criteria and guidelines

– Evaluate safety of alignment alternatives

– Technical studies

• Environmental impact study

• Traffic impact analysis

– Cost‐benefit analysis

Which Alternative Is Best?

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Alternatives Evaluation Stage• One Approach (no information on safety impact)

Existing Alt. 1Safety Compliance n.a. partial full **Operations Motorist delay veh-h/day 290 37 1872

"Cost"R.O.W. Property acquired acres 0 3 6Access Accessibility Lost access pts. 0 5 7.5Environment Area restored acres 0 1 2.3Construction Cost $/1000s 0 1000 1000

1.8Benefit/Cost Ratio

"Benefit"Impact

Impact Area Measure Units

Alternatives Evaluation Stage• Better Approach (with information on safety impact)

Existing Alt. 1Safety Crashes crash/yr 16 4 1200Operations Motorist delay veh-h/day 290 37 1872

"Cost"R.O.W. Property acquired acres 0 3 6Access Accessibility Lost access pts. 0 5 7.5Environment Area restored acres 0 1 2.3Construction Cost $/1000s 0 1000 1000

3.0Benefit/Cost Ratio

Impact Area Measure Units "Benefit"Impact

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Preliminary Design Stage

• Elements

– Preliminary design conference

• Identify complex or atypical conditions

– Preliminary design development

• Develop design to 30% level– Adhere to design criteria and guidelines

– Evaluate safety effect of “key” design elements

– Evaluate safety of features with complex conditions

– Design Exception Request

• Evaluate safety effect of deviation from criteria

Preliminary Design Stage• Key Design Elements that Influence Safety

– Design speed

– Lane width

– Shoulder width

– Median width and type

– Bridge width

– Structural capacity

– Horizontal alignment

– Vertical curvature

– Grade

– Stopping sight distance

– Cross slope

– Superelevation

– Vertical clearance

– Length of speed change lane

– Horizontal clearance

– Guardrail length

• Checked items can be evaluated using HSM

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Safety‐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

Point / Counterpoint

• Point

– Knowledge of a substandard road condition exposes an agency to a suit

– Accepting design exceptions, or not adding all known safety features, will expose an agency to suit

• Counterpoint

– Courts do not expect agencies to upgrade every road as conditions change

– Quantifying safety impacts and documenting findings shows prudence and yields defensible decisions

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Additional Resources

• AASHTO HSM Website

– http://www.highwaysafetymanual.org

• FHWA Highway Safety Manual Support

– http://safety.fhwa.gov/hsm/

• TRB Highway Safety Performance Committee

– http://www.safetyperformance.org

• FHWA CMF Clearinghouse

– http://www.cmfclearinghouse.org

Questions – Comments?

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James A. BonnesonKittelson & Associates, Inc.

July 2012

Urban Street Safety and Operation: Evaluating Performance Today and Tomorrow

OverviewOverview

• Background on Urban Street Evaluation• Urban Street Operation• Urban Street Safety• Combined Operation and Safety Evaluation• Urban Street Reliability Evaluation

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BackgroundBackground

• Observations– Newly released Highway Safety Manual (HSM) provides a means

to quantify road safety– Reliability of service (e.g., travel time reliability) has emerged as an

important descriptor of operational performance– Reliability is influenced by events (e.g., weather, crashes) that occur

over the year– Decisions to improve operation and reliability also influence safety– Decisions to improve safety also influence operation and reliability

• Presentation Basis– SHRP 2 Project L08: Incorporation of Travel Time Reliability into

the Highway Capacity Manual (HCM)

• Scope– Prediction (as opposed to measurement) to inform system design,

operation, and management

BackgroundBackground

• Reliability– Travel time distribution is used to define system reliability

• Distribution of performance during year• Reflects effect of safety and weather on operations• Large variability (i.e., spread) suggests an unreliable operation

IncidentWeather

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Urban Street OperationUrban Street Operation

• Traditional Operational Evaluation– Predictive methods used for evaluation

• Highway Capacity Manual• Synchro, etc.

– Analysis period• Design hour

– Predict performance measures for design hour– Assume: if proposed facility adequately serves design hour, then

it will provide adequate service during most other hours in year– Operation influenced by design and control choices

• Intersection control mode: signal vs. stop control• Left-turn mode: permitted, protected-permitted, protected-only• Change interval duration• Median type: raised curb, TWLTL, no median (undivided)• Turn bay presence

Urban Street OperationUrban Street Operation

• Operational Evaluation – HCM Approach– 2010 Highway Capacity Manual

• Urban street facilities• Urban street segments• Signalized intersections• Two-way stop-controlled intersections• All-way stop-controlled intersections• Roundabouts• Interchange ramp terminals

– Urban street segments performance measures• Travel time• Travel speed• Stop rate• Through delay

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Urban Street SafetyUrban Street Safety

• Traditional Safety Evaluation– Historically, safety prediction methods not available

• Highway Safety Manual is now available for safety prediction

– Safety audits used to subjectively identify safety issues

– Adherence to minimum criteria used to ensure nominal safety of alternatives


• Safety Evaluation – HSM Approach– Predictive method used for safety evaluation

• Highway Safety Manual

– Analysis period• One year

– Predict performance measures for whole year– Prediction for selected time periods during year not available

– Safety influenced by design and control choices• Intersection control mode: signal vs. stop control• Left-turn mode: permitted, protected-permitted, protected-only• Change interval duration• Median type: raised curb, TWLTL, no median (undivided)• Turn bay presence

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• Safety Evaluation – HSM Approach– 2010 Highway Safety Manual

• Urban street facilities• Urban street segments• Signalized intersections• Two-way stop-controlled intersections• All-way Stop-Controlled Intersections• Roundabouts• Interchange ramp terminals (forthcoming)

– Urban street performance measures• Expected crash frequency

– Fatal and injury, single vehicle– Fatal and injury, multiple vehicle– Property damage only, single vehicle– Property damage only, multiple vehicle

Combined Operation and Safety EvaluationCombined Operation and Safety Evaluation

• Consideration of Operations and Safety– Effect of design and control choices can be quantified

• Effect on operational performance• Effect on safety performance

– HSM makes combined, quantitative assessment possible

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• Issue 1 – Time Domain of Evaluation– Operations: design hour

– Safety: one year

– How well can the design hour reflect year-round performance?

Hour

Day of Year

00

24

365

Use HCM to estimate travel time for design hour.

Traditional Operational Evaluation

Hour

Day of Year

00

24

365

Use HSM to estimate expected crash frequency for year.

Safety Evaluation


• Issue 2 – Operations and Safety Not Independent– Design and traffic control decisions to improve operation...

• May also influence safety• Do not necessarily coincide with improvements in safety

– Design and traffic control decisions to improve safety...• May also influence operation• Do not necessarily coincide with improvements in operation

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• Whole-Year Analysis– A way to resolve these issues

– “Time domain” issue• Evaluate operations for every hour of the year

– Or, perhaps just selected hours of each day (e.g., pm peak period)

– “Interaction of operations and safety” issue• Quantify effect of crashes on operational performance

– Number of lanes closed– Duration of lane closure

• Quantify effect of congestion and speed on safety– Expected crash frequency– Proportion severe crashes


• Whole-Year Analysis– Computation time issue

• Evaluation of one, one-hour period takes fraction of a second• In “batch mode”, can evaluate all one-hour periods during one year in

just a few minutes

– Demand variation during year• Volume of each one-hour period is varied to reflect hour of day, day of

week, and month of year patterns

– Capacity variation during year• Capacity of each one-hour period is varied to reflect effect of external

events on safety and operation– Weather events– Crashes– Non-crash incidents (e.g., breakdown, debris in road, etc.)– Infrastructure failure (e.g., signal malfunction, dangerous potholes)– Train presence

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Hour

Day of Year

00

24

365

Use HCM to estimate travel time forevery hour of year.*

Whole-Year Operational Evaluation

Hour

Day of Year

00

24

365


Safety Evaluation

s

s

s

r

r

rr

r

r

• Whole-Year Analysis– Forecasting weather events

• Use historic weather records to determine...– Number of events per month– Probability of event (rain or snow) by month– Average Duration by month

• Use Monte-Carlo methods to randomly determine which days of month have an event, when it starts, when it ends, whether it is rain or snow

• Note: hours and location may change each year but travel time distribution for whole-year is fairly stable year after year


Hour

Day of Year

00

24

365



Hour

Day of Year

00

24

365


Safety Evaluation

P

P

P

P

P

F

PP

P

• Whole-Year Analysis– Forecasting crash events

• Use HSM to estimate expected crash frequency and severity

– Use Monte-Carlo methods to randomly determine which hours have a crash and where crash is located on street

• Procedure increases chance of crash during poor weather or high volume

– Note: hours and location may change each year but travel time distribution for whole-year is fairly stable year after year

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• Whole-Year Analysis - Summary– Evaluate every hour of interest during the year

– Predict effect of cashes on operations

– Predict effect of congestion and speed on crash frequency


• Whole-Year Analysis – One Last Thought– Feasible with 2010 HCM and 2010 HSM

• But, prediction of expected crash frequency for selected time periods during year not available (yet)

• So, we can’t get distribution of hour expected crash frequency (yet)

Hour

Day of Year

00

24

365



Hour

Day of Year

00

24

365


Safety Evaluation

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• Issue 3 – Work Zones and Special Events– Design and control decisions made to accommodate work zones

– When, and how long, to accommodate work zone also a factor

– All can be evaluated using whole-year analysis


• Payoff of Whole-Year Analysis– More complete “picture” of overall facility performance

• Distribution of performance during year• Reflects effect of safety and weather on operations

Lane‐Hours Blocked

Travel Time

Incidents per Day

IncidentWeather

Severe crash, many lanes

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• Payoff of Whole-Year Analysis– Implement strategies to reduce incidents or mitigate their impact

• Example: add shoulder (disabled vehicle refuge)– No change in operation during design hour, but– Average travel time decreases and operation is more reliable

Lane‐Hours BlockedTravel Time


• Payoff of Whole-Year Analysis– Implement strategies to increase speed and reduce delay

• Example: convert from protected-only to protected-permitted lefts– Travel time decreases during design hour (+ other hours), but– Crashes increase, – Average travel time is unchanged, and operation is less reliable

Travel Time Incidents per Day

Disclaimer: Trends are illustrative. One design or control change may not produce such dramatic distribution shifts—but a combination of changes could produce such shifts.

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• Questions or Comments?

Urban Street Reliability EvaluationUrban Street Reliability Evaluation

• SHRP 2 Research Project L08 Overview• Framework• Illustrative Analysis Capabilities

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Project L08 OverviewProject L08 Overview

• SHRP 2 Project L08• “Incorporation of Travel Time Reliability into the Highway

Capacity Manual”

• Objectives

– Determine how non-recurrent congestion impacts can be incorporated into HCM procedures

– Develop methodologies to predict travel time reliability on freeway facilities and urban streets

– Prepare a guide with that is suitable for potential inclusion in a future update of the HCM 2010

• Project schedule

– Ends in Fall 2012

FrameworkFramework

• Development Goals for Urban Streets– Quantify the effect of non-recurring congestion sources

• Weather• Demand variation• Incidents• Work zones• Special events

– Minimize the amount of required input data

– Assemble a set of nationally-representative default values

• Terms– Scenario – a unique combination of volume and traffic control

conditions for one analysis period (e.g., 1 hour)

– Study period – one or more consecutive scenarios during a day (e.g., 3-hour period from 3:30 pm to 6:30 pm)

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FrameworkFramework

• Work Flow– Start with the input data used to evaluate an urban street facility

using the 2010 HCM Urban Streets methodology• Enter the data in the urban streets engine and save it to a file• If desired, enter and save data for each work zone or special event

– Read the file and use it as a basis for scenario generation• Work day-by-day, analysis-period-by-analysis-period in chronologic

order through the year...– Predict weather events– Predict incident events– Adjust speed and saturation flow rate based on events– Adjust demand volumes using hourly, weekly, monthly factors– Save one revised file for each analysis period

– Submit each revised file to the urban streets engine

– Collect performance measures and compute reliability statistics

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Manual

Software

FrameworkFramework

• Flow Chart

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FrameworkFramework

• Input Data– Nearest city– Functional class– Analysis period duration (0.25 hr or 1.0 hr)– Study period duration (e.g., 7:00 am, extend for 3 hours)– Reliability reporting period (e.g., 1/1/2011, extend for 365 days)– Days of week considered (Su, M, Tu, W, Th, F, Sa)– Crash frequency by

• Segment• Intersection

– If work zone or special event present• Operating period (e.g., 4/1/2011, extend for 30 days)• Crash frequency adjustment factors

FrameworkFramework

• Work Zones and Special Events– Each is dealt with as a unique case for a unique time period

• Identify volume, geometry, and signal timing for each case– Include specific changes due to work zone or special event

» Lane closures» Alternative lane assignments» Special signal timing

• Create one urban streets engine input file for each case• Establish operating period (e.g., 4/1/2011, extend for 30 days)• Determine crash frequency adjustment factors

– Traffic demand changes• Not predicted• If analyst can estimate demand shifts, they can be reflected in

volumes entered in the input file

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Urban Streets Reliability EngineUrban Streets Reliability Engine

• Welcome

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Scenario GenerationScenario Generation

• Set Up

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• Set Up

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Performance SummaryPerformance Summary

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• Performance Measures– By Direction

• EB/NB• WB/SB

– By System Component• Facility• Segment

– By Performance Measure• Travel time• Travel speed• Stop rate• Running time• Through delay

• Examples– EB direction

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• Facility Travel Time (s) – PTI = 2.9

• Note– Maximum travel time

• 800 s

– Two crashes at same time but different intersections

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Summary StatisticsScenario evaluation interval: 1 Average: 120.38 5th percentile: 91.29Base free-flow speed, mi/h: 40.78 Standard deviation: 38.34 10th percentile: 92.45Base free-flow travel time, s: 60.20 Skewness: 7.72 80th percentile: 139.79

Median: 109.73 85th percentile: 143.76Number of obs.: 3120 95th percentile: 172.59

Planning Time


Summary StatisticsScenario evaluation interval: 1 Average: 21.52 5th percentile: 14.22Base free-flow speed, mi/h: 40.78 Standard deviation: 4.29 10th percentile: 16.06Base free-flow travel time, s: 60.20 Skewness: -0.58 80th percentile: 25.91

Median: 22.37 85th percentile: 26.25Number of obs.: 3120 95th percentile: 26.89

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ABCDEF• Facility Travel

Speed(mi/h) • Note

– 10 percent of analysis periods have LOS D, E, or F

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Urban Streets Reliability MethodologyUrban Streets Reliability Methodology

• Framework– Overview of next three parts

• Scenario Generation• Facility Evaluation• Performance Summary• Illustrative Analysis Capabilities

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Illustrative Analysis CapabilitiesIllustrative Analysis Capabilities

• Alternatives Analysis– Compare base condition to alternative condition

• Analysis Scenarios– Work zones and special events

• Alternative start dates and durations• Alternative lane closures and signal timing strategies

– Weather• Examine operational effects of strategies that reduce weather-

related crashes or crash severity (i.e., snow removal, resurfacing)– Incidents

• Examine operational effects of strategies that reduce incident duration

• Evaluate benefit of providing shoulder for stalled vehicle refuge– Design or Operation

• Evaluate alternative signal timing plans• Evaluate intersection lane allocations or segment geometry

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ClosureClosure

• Questions or Comments?

reliability guide for the hcm concepts...

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