14 Capacity Analysis

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Capacity AnalysisCapacity AnalysisCE 453 Lecture #15CE 453 Lecture #15


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ObjectivesObjectives

Review LOS definition and determinantsReview LOS definition and determinants

Define capacity and relate to idealDefine capacity and relate to idealcapacitiescapacities

Review calculating capacity using HCMReview calculating capacity using HCMprocedures for basic freeway sectionprocedures for basic freeway section

Focus on relations between capacity, levelFocus on relations between capacity, level--

ofof--service, and designservice, and design


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Level of Service (LOS)Level of Service (LOS)

ConceptConcept a qualitative measure describinga qualitative measure describingoperational conditions within a traffic streamoperational conditions within a traffic streamand their perception by drivers and/orand their perception by drivers and/or

passengerspassengersLevels represent range of operatingLevels represent range of operatingconditions defined by measures ofconditions defined by measures ofeffectiveness (MOE)effectiveness (MOE)


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LOS A (Freeway)LOS A (Freeway)

Free flow conditionsFree flow conditions

Vehicles areVehicles are

unimpeded in theirunimpeded in theirability to maneuverability to maneuverwithin the trafficwithin the trafficstreamstream

Incidents andIncidents andbreakdowns arebreakdowns areeasily absorbedeasily absorbed


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LOS BLOS BFlow reasonably freeFlow reasonably freeAbility to maneuverAbility to maneuveris slightly restrictedis slightly restrictedGeneral level ofGeneral level of

physical andphysical andpsychologicalpsychologicalcomfort provided tocomfort provided todrivers is highdrivers is high

Effects of incidentsEffects of incidentsand breakdowns areand breakdowns areeasily absorbedeasily absorbed


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LOS CLOS CFlow at or near FFSFlow at or near FFSFreedom to maneuverFreedom to maneuveris noticeablyis noticeablyrestrictedrestrictedLane changes moreLane changes moredifficultdifficultMinor incidents willMinor incidents willbe absorbed, but willbe absorbed, but willcause deteriorationcause deteriorationin servicein serviceQueues may formQueues may formbehind significantbehind significantblockageblockage


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LOS DLOS DSpeeds begin toSpeeds begin to

decline withdecline withincreasing flowincreasing flowFreedom to maneuverFreedom to maneuveris noticeably limitedis noticeably limited

Drivers experienceDrivers experiencephysical andphysical andpsychologicalpsychologicaldiscomfortdiscomfort

Even minor incidentsEven minor incidentscause queuing, trafficcause queuing, trafficstream cannot absorbstream cannot absorbdisruptionsdisruptions


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LOS ELOS E

CapacityCapacity

Operations are volatile,Operations are volatile,virtually no usable gapsvirtually no usable gapsVehicles are closelyVehicles are closelyspacedspaced

Disruptions such as laneDisruptions such as lanechanges can cause achanges can cause adisruption wave thatdisruption wave thatpropagates throughoutpropagates throughout

the upstream trafficthe upstream trafficflowflowCannot dissipate evenCannot dissipate evenminor disruptions,minor disruptions,incidents will causeincidents will causebreakdownbreakdown


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LOS FLOS F

Breakdown or forcedBreakdown or forcedflowflow

Occurs when:Occurs when: Traffic incidentsTraffic incidents

cause a temporarycause a temporaryreduction in capacityreduction in capacity

At points ofAt points ofrecurring congestion,recurring congestion,such as merge orsuch as merge orweaving segmentsweaving segments

In forecastIn forecastsituations, projectedsituations, projectedflow (demand)flow (demand)exceeds estimatedexceeds estimated

capacitycapacity


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Design Level of ServiceDesign Level of Service

This is the desired quality of traffic conditionsThis is the desired quality of traffic conditionsfrom a drivers perspective (used to determinefrom a drivers perspective (used to determinenumber of lanes)number of lanes)

Design LOS is higher for higher functionalDesign LOS is higher for higher functionalclassesclasses

Design LOS is higher for rural areasDesign LOS is higher for rural areas LOS is higher for level/rolling thanLOS is higher for level/rolling than

mountainous terrainmountainous terrain Other factors include: adjacent land use typeOther factors include: adjacent land use typeand development intensity, environmentaland development intensity, environmental

factors, and aesthetic and historic valuesfactors, and aesthetic and historic values Design all elements to same LOS (use HCM toDesign all elements to same LOS (use HCM to

analyze)analyze)


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Design Level of ServiceDesign Level of Service

(LOS)(LOS)


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CapacityCapacity DefinedDefinedCapacity: MaximumCapacity: Maximum hourly ratehourly rate ofofvehiclesvehicles or personsor personsthat canthat can reasonably bereasonably beexpectedexpected to pass a point,to pass a point, or traverse aor traverse a

uniform section of lane or roadwayuniform section of lane or roadway,,during a specified time periodduring a specified time period underunderprevailing conditionsprevailing conditions (traffic and roadway)(traffic and roadway)

Different for different facilitiesDifferent for different facilities

(freeway, multilane, 2(freeway, multilane, 2--lane rural, signals)lane rural, signals)Why would it be different?Why would it be different?


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Ideal CapacityIdeal Capacity

Freeways: CapacityFreeways: Capacity(Free(Free--Flow Speed)Flow Speed)

2,400 pcphpl (70 mph)2,400 pcphpl (70 mph)

2,350 pcphpl (65 mph)2,350 pcphpl (65 mph)2,300 pcphpl (60 mph)2,300 pcphpl (60 mph)


MultilaneMultilaneSuburban/RuralSuburban/Rural


2,100 (55 mph)2,100 (55 mph)2,000 (50 mph)2,000 (50 mph)

1,900 (45 mph)1,900 (45 mph)

22--lane rurallane rural 2,8002,800pcphpcph

SignalSignal 1,900 pcphgpl1,900 pcphgpl


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Principles for AcceptablePrinciples for Acceptable

Degree of Congestion:Degree of Congestion:1.1. Demand


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Multilane HighwaysMultilane Highways

Chapter 21 of the Highway Capacity ManualChapter 21 of the Highway Capacity Manual

For rural and suburban multilane highwaysFor rural and suburban multilane highways

Assumptions (Ideal Conditions, all otherAssumptions (Ideal Conditions, all otherconditions reduce capacity):conditions reduce capacity): Only passenger carsOnly passenger cars

No direct access pointsNo direct access points

A divided highwayA divided highway FFS > 60 mphFFS > 60 mph

Represents highest level of multilane rural andRepresents highest level of multilane rural andsuburban highwayssuburban highways


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Multilane HighwaysMultilane Highways

Intended for analysis of uninterruptedIntended for analysis of uninterrupted--flow highway segmentsflow highway segments

Signal spacing >2

.0 milesSignal spacing >2

.0 miles No onNo on--street parkingstreet parking

No significant bus stopsNo significant bus stops

No significant pedestrian activitiesNo significant pedestrian activities


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Source: HCM, 2000


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Step 1: Gather data

Step 2: Calculate capacity

(Supply)


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Source: HCM, 2000


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Source: HCM, 2000

Lane WidthLane Width

Base Conditions: 12 foot lanesBase Conditions: 12 foot lanes


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Source: HCM, 2000

Lane Width (Example)Lane Width (Example)

How much does use of 10-foot lanes decreasefree flow speed?

Flw = 6.6 mph


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Lateral ClearanceLateral Clearance

Distance to fixed objectsDistance to fixed objects

AssumesAssumes

>= 6 feet from right edge of travel lanes to>= 6 feet from right edge of travel lanes toobstructionobstruction

>= 6 feet from left edge of travel lane to>= 6 feet from left edge of travel lane toobject in medianobject in median

Source: HCM, 2000


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Lateral ClearanceLateral Clearance

TLC = LCTLC = LCRR + LC+ LCLL

TLC = total lateral clearance in feetTLC = total lateral clearance in feet

LCLCRR = lateral clearance from right edge of= lateral clearance from right edge oftravel lanetravel lane

LCLCLL= lateral clearance from left edge of= lateral clearance from left edge of

travel lanetravel lane

Source: HCM, 2000


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Source: HCM, 2000


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Example: Calculate lateral clearance adjustment for a 4-lane

divided highway with milepost markers located 4 feet to the

right of the travel lane.

TLC = LCR+ LCL = 6 + 4 = 10

Flc = 0.4 mph Source: HCM, 2000


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fm: Accounts for friction between opposing directions of

traffic in adjacent lanes for undivided

No adjustment for divided, fm = 1

Source: HCM, 2000


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Fa accounts for interruption due to access points along

the facility

Example: if there are 20 access points per mile, what is

the reduction in free flow speed?

Fa

= 5.0 mph


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Estimate Free flow SpeedEstimate Free flow Speed

BFFS = free flow under ideal conditions

FFS = free flow adjusted for actual conditions

From previous examples:

FFS = 60 mph 6.6 mph - 0.4 mph 0 5.0 mph =

48 mph ( reduction of 12 mph)


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Source: HCM, 2000

Step 3: Estimate

demand


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Calculate Flow Rate


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Heavy Vehicle AdjustmentHeavy Vehicle Adjustment

Heavy vehicles affect trafficHeavy vehicles affect trafficSlower, largerSlower, largerffhvhv increases number of passenger vehicles toincreases number of passenger vehicles to

account for presence of heavy trucksaccount for presence of heavy trucks


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f(hv) General Grade Definitions:f(hv) General Grade Definitions:

Level: combination of alignment (horizontal andLevel: combination of alignment (horizontal andvertical) that allows heavy vehicles to maintainvertical) that allows heavy vehicles to maintainsame speed as pass. cars (includes short gradessame speed as pass. cars (includes short grades2% or less)2% or less)

Rolling: combination that causes heavy vehiclesRolling: combination that causes heavy vehiclesto reduce speed substantially below P.C. (but notto reduce speed substantially below P.C. (but notcrawl speed for any length)crawl speed for any length)

Mountainous: Heavy vehicles at crawl speed forMountainous: Heavy vehicles at crawl speed for

significant length or frequent intervalssignificant length or frequent intervals Use specific grade approach if grade less thanUse specific grade approach if grade less than

3% is more than mile or grade more than 3% is3% is more than mile or grade more than 3% ismore than mile)more than mile)


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Example: for 10% heavy trucks on rollingterrain, what is Fhv?

For rolling terrain, ET = 2.5

Fhv = _________1_______ = 0.87

1 + 0.1 (2.5 1)


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Driver Population Factor (fDriver Population Factor (fpp))

NonNon--familiar users affect capacityfamiliar users affect capacity

ffpp = 1, familiar users= 1, familiar users

1 > f1 > fpp >=0.85, unfamiliar users>=0.85, unfamiliar users


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Source: HCM, 2000

Step 4: Determine

LOS

Demand Vs.

Supply


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Calculate vCalculate vpp

Example:Example: base volume is 2,500 veh/hourbase volume is 2,500 veh/hour

PHF = 0.9, N = 2PHF = 0.9, N = 2ffhvhv from previous, ffrom previous, fhvhv = 0.87= 0.87

NonNon--familiar users, ffamiliar users, fpp = 0.85= 0.85

vp = _____2,500 vph_____ = 1878 pc/ph/pl

0.9 x 2 x 0.87 x 0.85


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Calculate DensityCalculate Density

Example: for previous

D = _____1878 vph ____ =39.1 pc/mi/lane

48 mph


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LOS = E

Also, D = 39

.1 pc/mi/ln, LOS E


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Design DecisionDesign Decision

What can we change in a design toWhat can we change in a design toprovide an acceptable LOS?provide an acceptable LOS?

Lateral clearance (only 0.4 mph)Lateral clearance (only 0.4 mph)Lane widthLane width

Number of lanesNumber of lanes


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Source: HCM, 2000

Lane Width (Example)Lane Width (Example)

How much does use of 10 foot lanes decrease freeflow speed?

Flw = 6.6 mph


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Recalculate DensityRecalculate Density

Example: for previous (but with wider lanes)

D = _____1878 vph ____ =34.1 pc/mi/lane

55 mph


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LOS = E

Now D = 34.1 pc/mi/ln, on border of LOS E


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Recalculate vRecalculate vpp, while adding a lane, while adding a lane

Example:Example: base volume is 2,500 veh/hourbase volume is 2,500 veh/hour

PHF = 0.9, N = 3PHF = 0.9, N = 3ffhvhv from previous, ffrom previous, fhvhv = 0.87= 0.87

NonNon--familiar users, ffamiliar users, fpp = 0.85= 0.85

vp = _____2,500 vph_____ = 1252 pc/ph/pl

0.9 x 3 x 0.87 x 0.85


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Calculate DensityCalculate Density

Example: for previous

D = _____1252 vph ____ =26.1 pc/mi/lane

48 mph


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LOS = D

Now D = 26 1 pc/mi/ln LOS D (almost C)

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14 Capacity Analysis