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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 “ideal” Define capacity and relate to “ideal”

capacitiescapacities Review calculating capacity using HCM Review calculating capacity using HCM

procedures for basic freeway sectionprocedures for basic freeway section Focus on relations between capacity, Focus on relations between capacity,

level-of-service, and designlevel-of-service, and design

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Level of Service (LOS)Level of Service (LOS)Concept – a qualitative measure Concept – a qualitative measure describing operational conditions within a describing operational conditions within a traffic stream and their perception by traffic stream and their perception by drivers and/or passengers drivers and/or passengers Levels represent range of operating Levels represent range of operating conditions defined by measures of conditions defined by measures of effectiveness (MOE)effectiveness (MOE)

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LOS A (Freeway)LOS A (Freeway)Free flow conditionsFree flow conditionsVehicles are Vehicles are unimpeded in their unimpeded in their ability to maneuver ability to maneuver within the traffic within the traffic streamstreamIncidents and Incidents and breakdowns are breakdowns are easily absorbedeasily absorbed

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LOS BLOS BFlow reasonably freeFlow reasonably freeAbility to maneuver is Ability to maneuver is slightly restrictedslightly restrictedGeneral level of General level of physical and physical and psychological comfort psychological comfort provided to drivers is provided to drivers is highhighEffects of incidents and Effects of incidents and breakdowns are easily breakdowns are easily absorbedabsorbed

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LOS CLOS CFlow at or near FFSFlow at or near FFSFreedom to maneuver Freedom to maneuver is noticeably restrictedis noticeably restrictedLane changes more Lane changes more difficultdifficultMinor incidents will be Minor incidents will be absorbed, but will cause absorbed, but will cause deterioration in servicedeterioration in serviceQueues may form Queues may form behind significant behind significant blockageblockage

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LOS DLOS DSpeeds begin to decline Speeds begin to decline with increasing flowwith increasing flowFreedom to maneuver is Freedom to maneuver is noticeably limitednoticeably limitedDrivers experience Drivers experience physical and physical and psychological discomfortpsychological discomfortEven minor incidents Even minor incidents cause queuing, traffic cause queuing, traffic stream cannot absorb stream cannot absorb disruptionsdisruptions

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LOS ELOS ECapacityCapacityOperations are volatile, Operations are volatile, virtually no usable gapsvirtually no usable gapsVehicles are closely spacedVehicles are closely spacedDisruptions such as lane Disruptions such as lane changes can cause a changes can cause a disruption wave that disruption wave that propagates throughout the propagates throughout the upstream traffic flowupstream traffic flowCannot dissipate even minor Cannot dissipate even minor disruptions, incidents will disruptions, incidents will cause breakdowncause breakdown

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LOS FLOS FBreakdown or forced flowBreakdown or forced flowOccurs when:Occurs when: Traffic incidents cause a Traffic incidents cause a

temporary reduction in temporary reduction in capacitycapacity

At points of recurring At points of recurring congestion, such as merge congestion, such as merge or weaving segmentsor weaving segments

In forecast situations, In forecast situations, projected flow (demand) projected flow (demand) exceeds estimated exceeds estimated capacitycapacity

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Design Level of ServiceDesign Level of ServiceThis is the desired quality of traffic conditions from a This is the desired quality of traffic conditions from a driver’s perspective (used to determine number of driver’s perspective (used to determine number of lanes)lanes)

Design LOS is higher for higher functional classesDesign LOS is higher for higher functional classes Design LOS is higher for rural areasDesign LOS is higher for rural areas LOS is higher for level/rolling than mountainous LOS is higher for level/rolling than mountainous

terrainterrain Other factors include: adjacent land use type and Other factors include: adjacent land use type and

development intensity, environmental factors, development intensity, environmental factors, and aesthetic and historic valuesand aesthetic and historic values

Design all elements to same LOS (use HCM to Design all elements to same LOS (use HCM to analyze)analyze)

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

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Capacity – DefinedCapacity – Defined   Capacity: Maximum Capacity: Maximum hourly ratehourly rate of of vehicles vehicles or persons or persons that can that can reasonably be expectedreasonably be expected to pass a point, to pass a point, or traverse a uniform section of or traverse a uniform section of lane or roadwaylane or roadway, during a specified , during a specified time period time period under under prevailing conditionsprevailing conditions (traffic and roadway)(traffic and roadway)Different for different facilities (freeway, Different for different facilities (freeway, multilane, 2-lane rural, signals)multilane, 2-lane rural, signals)Why would it be different? Why would it be different?

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Ideal CapacityIdeal CapacityFreeways: Capacity Freeways: Capacity (Free-Flow Speed)(Free-Flow Speed)2,400 pcphpl (70 2,400 pcphpl (70 mph)mph)2,350 pcphpl (65 2,350 pcphpl (65 mph)mph)2,300 pcphpl (60 2,300 pcphpl (60 mph)mph)2,250 pcphpl (55 2,250 pcphpl (55 mph)mph)

Multilane Multilane Suburban/RuralSuburban/Rural2,200 pcphpl (60 mph)2,200 pcphpl (60 mph)2,100 (55 mph)2,100 (55 mph)2,000 (50 mph)2,000 (50 mph)1,900 (45 mph)1,900 (45 mph)

2-lane rural – 2,800 2-lane rural – 2,800 pcphpcphSignal – 1,900 pcphgplSignal – 1,900 pcphgpl

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Principles for Acceptable Principles for Acceptable Degree of Congestion:Degree of Congestion:

1.1. Demand <= capacity, even for short timeDemand <= capacity, even for short time2.2. 75-85% of capacity at signals75-85% of capacity at signals 3.3. Dissipate from queue @ 1500-1800 vphDissipate from queue @ 1500-1800 vph 4.4. Afford some choice of speed, related to trip Afford some choice of speed, related to trip

lengthlength5.5. Freedom from tension, esp long trips, < 42 Freedom from tension, esp long trips, < 42

veh/mi.veh/mi. 6.6. Practical limits - users expect lower LOS in Practical limits - users expect lower LOS in

expensive situations (urban, mountainous)expensive situations (urban, mountainous)

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Multilane HighwaysMultilane HighwaysChapter 21 of the Highway Capacity ManualChapter 21 of the Highway Capacity ManualFor rural and suburban multilane highwaysFor rural and suburban multilane highwaysAssumptions (Ideal Conditions, all other Assumptions (Ideal Conditions, all other conditions reduce capacity):conditions reduce capacity): Only passenger carsOnly passenger cars No direct access points No direct access points A divided highwayA divided highway FFS > 60 mphFFS > 60 mph Represents highest level of multilane rural and Represents highest level of multilane rural and

suburban highwayssuburban highways

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Multilane HighwaysMultilane HighwaysIntended for analysis of Intended for analysis of uninterrupted-flow highway uninterrupted-flow highway segmentssegments Signal spacing > 2.0 milesSignal spacing > 2.0 miles No on-street parkingNo on-street parking No significant bus stopsNo significant bus stops No significant pedestrian activitiesNo significant pedestrian activities

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Source: HCM, 2000

1818Source: HCM, 2000

Step 1: Gather data

Step 2: Calculate capacity (Supply)

1919Source: HCM, 2000

2020Source: HCM, 2000

2121Source: HCM, 2000

Lane WidthLane WidthBase Conditions: 12 foot lanesBase Conditions: 12 foot lanes

2222Source: HCM, 2000

Lane Width (Example)Lane Width (Example)

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

Flw = 6.6 mph

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Lateral ClearanceLateral ClearanceDistance to fixed objectsDistance to fixed objectsAssumesAssumes >= 6 feet from right edge of travel lanes >= 6 feet from right edge of travel lanes

to obstructionto obstruction >= 6 feet from left edge of travel lane >= 6 feet from left edge of travel lane

to object in medianto object in median

Source: HCM, 2000

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Lateral ClearanceLateral ClearanceTLC = LCTLC = LCRR + LC + LCLL

TLC = total lateral clearance in feetTLC = total lateral clearance in feetLCLCRR = lateral clearance from right edge of = lateral clearance from right edge of

travel lanetravel laneLCLCLL= lateral clearance from left edge of = lateral clearance from left edge of

travel lanetravel lane

Source: HCM, 2000

2525Source: 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 conditionsFFS = free flow adjusted for actual conditionsFrom previous examples:

FFS = 60 mph – 6.6 mph - 0.4 mph – 0 – 5.0 mph = 48 mph ( reduction of 12 mph)

3030Source: HCM, 2000

Step 3: Estimate demand

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Calculate Flow Rate

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Heavy Vehicle AdjustmentHeavy Vehicle AdjustmentHeavy vehicles affect trafficHeavy vehicles affect trafficSlower, largerSlower, largerffhvhv increases number of passenger vehicles to account for presence of increases number of passenger vehicles to account for presence of heavy trucksheavy trucks

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f(hv) General Grade Definitions:f(hv) General Grade Definitions: Level: combination of alignment (horizontal Level: combination of alignment (horizontal

and vertical) that allows heavy vehicles to and vertical) that allows heavy vehicles to maintain same speed as pass. cars (includes maintain same speed as pass. cars (includes short grades 2% or less)short grades 2% or less)

Rolling: combination that causes heavy Rolling: combination that causes heavy vehicles to reduce speed substantially below vehicles to reduce speed substantially below P.C. (but not crawl speed for any length)P.C. (but not crawl speed for any length)

Mountainous: Heavy vehicles at crawl speed Mountainous: Heavy vehicles at crawl speed for significant length or frequent intervalsfor significant length or frequent intervals

Use specific grade approach if grade less Use specific grade approach if grade less than 3% is more than ½ mile or grade more than 3% is more than ½ mile or grade more than 3% is more than ¼ mile)than 3% is more than ¼ mile)

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Example: for 10% heavy trucks on rolling terrain, 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))Non-familiar users affect capacityNon-familiar users affect capacityffpp = 1, familiar users = 1, familiar users1 > f1 > fpp >=0.85, unfamiliar users >=0.85, unfamiliar users

3636Source: HCM, 2000

Step 4: Determine LOS

Demand Vs. Supply

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Calculate vCalculate vpp

Example:Example: base volume is 2,500 veh/hour base volume is 2,500 veh/hourPHF = 0.9, N = 2PHF = 0.9, N = 2ffhvhv from previous, f from previous, fhvhv = 0.87 = 0.87Non-familiar users, fNon-familiar 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 DecisionWhat can we change in a design to What can we change in a design to provide an acceptable LOS?provide an acceptable LOS?Lateral clearance (only 0.4 mph) Lateral clearance (only 0.4 mph) Lane widthLane widthNumber of lanesNumber of lanes

4141Source: HCM, 2000

Lane Width (Example)Lane Width (Example)

How much does use of 10 foot lanes decrease free flow 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/hour base volume is 2,500 veh/hourPHF = 0.9, N = 3PHF = 0.9, N = 3ffhvhv from previous, f from previous, fhvhv = 0.87 = 0.87Non-familiar users, fNon-familiar 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)