highway capacity lecture 1&2

8/9/2019 Highway Capacity Lecture 1&2

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Learning Objectives

To define basic concepts relating to highwaycapacity analysis

To apply capacity and LOS analysis forfreeways(Chapter 7.1 ~ 7.3; HCM Ch23)

Dr. Rakesh Kumar, Assistant Professor, SVNIT


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Transportation Facilities

Uninterrupted-flow facilities

no fixed elements external to the traffic streamto interrupt flow

Freeways, multilane hwy, two-lane hwy Interrupted-flow facilities

have controlled and uncontrolled access pointsthat can interrupt flow

(un) signalized intersections, urban streets

Performance?


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Capacity Analysis

Procedures for estimating the traffic-carrying ability of segments or points ofa facility over a range of defined

operational conditions

Purposes

Assessing facility performance

Planning and designing improved facilities


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Capacity

maximum hourly rate at which persons orvehicles reasonably can be expected to traverse apoint or a uniform section of a lane or roadway

during a given time period under prevailingroadway, traffic, and control conditions

(HCM 2000)


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Capacity

maximum hourly rate at which persons orvehicles reasonably can be expected totraverse a point or a uniform section of a laneor roadway during a given time period underprevailing roadway, traffic, and controlconditions (HCM 2000)

Base conditions

good weather

good pavement conditions

users familiar with the facility

no impediments to traffic flow


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Level of Service (LOS)

A qualitative measure describingoperational conditions in a traffic streamand their perception by Users/motorists

Range A (best) through F (worst) Determined by measure of effectiveness

(MOE) such as speed and travel time,freedom to maneuver, traffic interruptions,

and comfort and convenience


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Level of Service (freeways)

LOS A LOS B LOS C

LOS D LOS E LOS F


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Service Flow Rates

Maximum flow rate attainable ateach level of service (except for LOSF) for a given facility

Define the flow boundaries betweenlevels of service

Hourly service flow rate is defined asfour times the peak 15-min volume


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Capacity Analysis forFreeways

Only for basic freeway segments

Where there are no interruptions to theflow of traffic

Outside the area of influence of freewayweaving areas and freeway ramp

junctions


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Capacity Analysis forFreeways

Base (ideal) conditions

Minimum lane widths of 12 ft

Minimum right-shoulder lateral clearance of 6 ft

Minimum median lateral clearance of 2 ft 100% passenger cars

Five or more lanes for one direction

Interchange spacing at 2 mi or greater

Level terrain (grades no greater than 2%)

Drivers primarily regulars


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LOS for Freeways

Defined by density (k)

A: k 11 (Smin =480 ft), free flow

B: k 18 (Smin =290 ft), reasonably free flow

C: k 26 (Smin =200 ft), nearly free flow speed;maneuvers limited; reduced comfort

D: k 35 (Smin =150 ft), reduced speeds; limited

freedom to maneuver; potential for queuing

E: k 45 (Smin =120 ft), at capacity; unstable flow F: extensive queuing behind breakdown pts


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LOS Criteria

Table 7-1, p.229

Volume-to-capacity

ratio


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Speed-flow Curves and LOS forBasic Freeway Segments

Fig 7-3, p.230

K = q/v

Capacity


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Freeway Capacity Analysis

Determine flow rate

Determine free flow speed

Determine level of service


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Determining Flow Rate

Adjusting hourly volumes or estimates, typicallyreported in veh/h, to arrive at an equivalentpassenger-car flow rate in passenger cars per hour(pc/h)


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Peak Hour Factor (PHF)

Accounting for variation in traffic flowwithin an hour

On freeways, typically ranging from 0.80(rural/off-peak) to 0.95 (urban)

If possible, field data should be used


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Heavy Vehicle Adjustment Factor

Accounting for the presence of heavyvehicles (trucks, buses, RVs)


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Passenger-Car Equivalents

Values of ET

and ER

selected for one of

the following conditions:

Extended freeway segments

Specific grades

Downgrades


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Extended freeway segmentsno one grade of 3% or greater is longer than0.25 mi or no one grade of less than 3% is

longer than 0.5 mi

Level: includes short grades of no more than 2%, HV operate at thesame speed as PC

Rolling: HV substantially slower than PC but do not operate at crawlspeeds for significant length of time

Mountainous: HV operate at crawl speeds for significant length of time


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Specific Gradesgrade of more than 0.5 mi for grades less than3% or grade of more than 0.25 mi for grades of

3% or more

Table 7-3, p.232


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Specific Grades

Table 7-4, p.233


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Downgradeswhen trucks must often use low gears to avoidgaining too much speed and running out of

control

Table 7-5, p.233


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Driver Population Factor

Accounting for the presence of unfamiliarusers

Range from 0.85 to 1.00

Use 1.00 unless evidence suggestsotherwise


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Determine flow rate




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Determining FFS

FFS: mean speed of passenger carsmeasured during low to moderate flows

Two methods:

Field measurement Weekday off-peak hours

No adjustment needed

Estimated based on physical characteristics of

the freeway segment


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Estimating FFS


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Estimating FFS

Adjustment for lane width


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Estimating FFS

Adjustment for lateral clearance


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Estimating FFS

Adjustment for number of lanes


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Estimating FFS

Adjustment for interchange density


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Determine flow rate




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Determining LOS

Based on the equivalent passenger-carflow rate (Vp) and adjusted free-flow speed

(S)

Step 1


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Determining LOS

Step 2


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Practice Problem

BFFS = 70 mph (LOS A or B)

N = 3, LC = 3ft, Lane Width = 11ft

1 interchange/mi

V = 3080 veh/hr (1-dir)

PHF = 0.88

154 Tr & Buses

Level terrain All commuters

Determine density (D) and LOS


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Housekeeping

Exam I grades available; seeGhazan for question sheets

Entire Chapter 7 (K&L) and HCM Ch23

Homework Assignment #4 due at1:20pm next Wed, Oct 24


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Highway Capacity (II)

Oct 29, 2009


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Learning Objectives

To perform capacity analysis for multilanehighways

To perform capacity analysis for two-lanehighways

To distinguish differences in analysis methodsfor freeway/multilane highway/two-lanehighway

(Chapter 7; HCM Ch23)


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Practice Problem

BFFS = 70 mph (LOS A orB)

N = 3, LC = 3ft, LW =11ft

1 interchange/mi V = 3080 veh/hr (1-dir)

PHF = 0.88

154 Tr & Buses

Level terrain

All commuters

Determine density (D)and LOS


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Freeway AnalysisApplications

Operational analysis

Given geometric and traffic conditions, estimatespeed, density, and LOS

Design Given forecasted demand volume and desired

LOS, determine number of lanes needed

Planning

Given forecasted AADT (annual average dailytraffic), HV%, and terrain type, estimateoperational and design parameters


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Freeway AnalysisApplications

Planning (cont.)

First step: convert AADT to DDHV(directional design hour volume)

K: proportion of AADT occurring in peak hour

0.08-0.09 (urban), 0.15-0.20 (rural)D: proportion of peak hour traffic in the heavier

direction

0.52 (urban circumferential fwy),

0.55 (urban radial fwy), 0.75 (rural fwy)

DKAADTDDHV dayvehhrveh )/()/(


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Multilane Highways

4 to 6 lanes

Often with physical medians or two-way left-turn lanes

Posted speed limit of of 40~55mph

Flow not as efficient as freeways


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Multilane Highways


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Multilane Highway Analysis

Minor differences in base conditions Minimum lane widths of 12 ft

Objects no closer than 6 ft from the edge of the traveled pavement(at the roadside or median)

No direct access points

Divided highway

FFS of 60mph pr more

100% passenger cars

Drivers primarily regulars

Similar to freeways Determine flow rate


Determine level of service defined by density


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Same as freeway analysis


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Determining FFS

Two methods:

Direct measurement Low-volume

conditions (below

1400 pc/h/ln) No adjustment

needed

Use a baseline andadjust it

Use BFFS of 60 mphor estimate based onthe 85th percentilespeed

AMLCLW ffffBFFSFFS fLW

=adjustment for lane width(mph)

fLC=adjustment for right-shoulder lateral clearance(mph)

fM=adjustment for median type(mph)

fA=adjustment for accesspoints (mph)

L Width Adj t t


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Lane Width Adjustment(fLW )

Base condition (fLW = 0)

Average width of 12 ft. or wider across alllanes

From Highway Capacity Manual, 2000

Same as Freeway LOS

AMLCLW ffffBFFSFFS

Table 7-11

L t l Cl


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Lateral ClearanceAdjustment (fLC )

Base condition (fLC = 0)

12 ft or greater TLC


LRLCLCTLC +=

AMLCLW ffffBFFSFFS

Table 7-12

Multilane Highway LOS


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Median Adjustment (fM)

Base condition (fM = 0)

Divided highway


AMLCLW ffffBFFSFFS

Table 7-13

A P i t D itMultilane Highway LOS


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Access-Point DensityAdjustment (fA)

Base condition (fA = 0)

0 access points per mile

For NAPM 40: fA = 0.25 #access point/mi

For NAPM > 40: fA = 10

Default density give by Table 7-15


AMLCLW ffffBFFSFFS

Table 7-14


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Determining LOS

Same procedure as forfreeways

Use vp and FFS curve to

find average passenger

car speed (S) Calculate density

Compare calculateddensity with LOS Criteriafor Basic Freeway

Segments (Table 7-10) orspeed-flow curve (figure 7-5)

Figure 7-5


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Two-lane Highways


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Two-lane Highways

Undivided two-lane, two-way highways

Class I consistent high-speed operations and

infrequent passing delays Long-distance trips

LOS defined by % time-spent-following andavg. travel speed

Class II Short trips; accessibility

LOS defined by % time-spent-following only


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Two-lane Highway Analysis

Base conditions Minimum lane widths of 12 ft

Shoulders at least 6 ft

No No Passing Zones on the highway

No traffic control or turning vehicles Level terrain

100% passenger cars

50/50 split

Either as two-way segments with bothdirections combined or as directionalsegments with each direct consideredseparately


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Two-way Segment Analysis

Determine flow rate (iterativeprocedure)

HVG

pffPHP

Vv

=

fHV = Adjustment for heavy vehicle

Same equation, different tables (7-27, 7-28)

fG= Adjustment for grade

Table 7-25 for speed estimation, 7-26

for % time-spent-following estimation


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Determine free-flow speed: two methods

Direct measurement

Use a baseline and adjust it

HV

f

FM f

V

SFFS 00776.0

ALS ffBFFSFFS


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Determine average travel speed

Determine PTSF

npp fvFFSATS 00776.0

( )pvAnpdeBPTSF fBPTSFPTSF 000879.0 /1100 +fnp = Adjustment for % of no-passing zones

fd/np = Adjustment for directional distribution and

% of no-passing zones


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Determine LOS


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Determine LOS


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Determine LOS

Directional Segment


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Directional SegmentAnalysis

Section 5.3 in chapter 7

highway capacity lecture 1&2

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