Capacity and LOS for Uninterrupted Flow_IITKGP

8/2/2019 Capacity and LOS for Uninterrupted Flow_IITKGP

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Indian Institute of TechnologyKharagpur

Traffic EngineeringTraffic Engineering --CE 41625CE 41625

Capacity and LOS AnalysisCapacity and LOS AnalysisUninterrupted Flow FacilitiesUninterrupted Flow Facilities



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CAPACITYCAPACITY

The maximum rate of flow on a road section

or lane group that can

be expected to occur under prevailing

conditions:

Traffic Geometrics

Control



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LEVEL OF SERVICELEVEL OF SERVICEAn indication of the

quality of thedriving

experience on atransportation

facility (in terms

of speed, freedomto maneuver,

comfort & safety)

speed, u

flow, q

A

BC

D

E

F



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LOS & CapacityLOS & Capacity

Note that for uninterrupted flow facilitytypes, capacity (max. flow, qmax) is defined

as the max. flow value for level of service E

This is not the case for interrupted flow

facilities (e.g. controlled intersections)



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Cases of Uninterrupted FlowCases of Uninterrupted FlowConsidered in the HCMConsidered in the HCM

Freeways – Basic Freeway Sections

– Weaving Areas

– Ramp and Ramp Junctions

– Overall Freeway Facilities

Rural and Suburban Highways – Multi-Lane Highways

– Two-Lane Highways



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Cases of Interrupted Flow &Cases of Interrupted Flow &Misc. Considered in the HCMMisc. Considered in the HCM

Signalized Intersections

Unsignalized Intersections

Urban Streets Ramp Interchanges

Transit Capacity

Pedestrians

Bicycles



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Quantifying LOS for Major Quantifying LOS for Major Highway Facility TypesHighway Facility Types

Freeways, Multi-lane Highways, Ramp Junctions

& Weaving Sections

– LOS Based on Density (mostly)

Two-Lane Highways

– LOS Based on “Percent-Time-Spent Following” &

Average Speed

Controlled Intersections and Interchange Areas

– LOS Based on Average Control Delay per Vehicle



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Quantifying LOS for Other Quantifying LOS for Other Facility & Service TypesFacility & Service Types

Urban Streets – LOS Based on Average Travel Speed

Bike Facilities

– Bike Paths – LOS Based on Events per Hour – Bike Lanes – LOS Based on Average Travel Speed

Pedestrians

– LOS Based on Density

Transit

– Scheduled: Service Frequency; Hours of Service;Loading Density; Reliability (on-time performance)

– Paratransit: Time to Access (Waiting Time)



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Minimum LOS Design TargetsMinimum LOS Design Targets

Arterials

– C or D for urban/suburban conditions

– B or C for rural conditions

Collectors – C for most rural conditions

– D for urban and mountainous rural conditions

Local Roads

– D for all conditions



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Basic Freeway SegmentsBasic Freeway SegmentsHCM MethodologyHCM Methodology

Determines the capacity/LOS for freewaysegments outside the influence of ramps andweaving (potential bottlenecks for whichdifferent analysis methodologies exist)

The analysis methodology considers eachdirection of traffic separately

Three problem types are common: – Given volumes & facility design, find LOS

– Given facility and target LOS, find max volume

– Given volumes & target LOS, design the facility



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Levels of Levels of Service for Service for

Basic FreewayBasic FreewaySectionsSections

A: Free flow B: High speed, some restrictions to lane changing

C: Freedom of maneuver is restricted

D: Speeds decline with increasing flows E: No gaps, little room to maneuver

F: Breakdown conditions - slow, bumper to bumper

(Note: lower part of u-q curve, which is not shown)



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Service Flow RateService Flow Rate

The maximum 15 min. flow rate (vph) that a

highway can accommodate under prevailing

conditions at a given level of service.

⇒ SF SF ...SF are defined for each highwayA B F, ,



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Prevailing ConditionsPrevailing Conditions

Lane width and lateral obstructions

Traffic composition

Grade and length of grade

G

L

G

L



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Formula for Service Flow RateFormula for Service Flow Rate

SF MSF N f f i i HV p= × × ×

Design Parameter

From Tables

MSFi = maximum service flow rate for LOS i (for a givenfree flow speed - in passenger cars per hour per lane

“pcphpl”, for ideal conditions)

N = number of lanes

f HV = adjustment factor for heavy vehicles (≤1)

f p = driver population factor (usually =1 but can drop to

0.85 if local data shows unusually inefficient driving)



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Key LOS Parameter ValuesKey LOS Parameter Values



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Visualizing Freeway LOSVisualizing Freeway LOSLOS A: LOS E:

LOS C: LOS F:

Source (videos): HCM 2000



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Secondary Analysis FormulaSecondary Analysis Formula

Where:

v = equivalent 15 min. flow rate (pcphpl) used toenter Exh. 23-2 to determine LOS

V = hourly volume (design hourly vol.) (vph)

PHF = peak hour factor

N = number of lanes

f HV = heavy vehicle factor (≤1)

f p

= population factor (rarely less than 1)

))()()(( p HV f f N PHF

V v =



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ThatThat’’s Pretty Much It!s Pretty Much It!

Except for two important loose ends: – Where do we get free flow speed (FFS)?

– How do we determine the heavy vehicle factor (f HV)?



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Determining Free Flow SpeedDetermining Free Flow Speed Measure directly in the field (preferred)

Measure directly at a similar facility (2

nd

best) Use HCM estimation method:

Where:

– BFFS = Base free flow speed (from rule of thumb)

– f LW = Lane width adjustment factor (from table) – f LC = Lateral clearance adjustment factor (“)

– f N = Adjustment factor for the number of lanes (“)

– f ID = Adjustment factor for interchange density (“)

ID N LC LW

f f f f BFFS FFS −−−−=



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Heavy Vehicle Factors (Heavy Vehicle Factors (f f hvhv)) Considers two types: trucks/buses and RVs

Three Cases: – Extended Freeway Segment

Terrain classified as level, rolling or mountainous

– Specific Upgrade Segment Grade ≥3% and length of grade > 1/4 mile, or

Grade 2-3% and length of grade > 1/2 mile

– Specific Downgrade Segment

Grade ≥4% and length of grade > 4 miles The approach is the same in all cases, all that

changes is the table where one gets passenger car equivalence (PCE) factors



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f f hvhv for Extended Segmentsfor Extended Segments Level, Rolling, Mountainous selected based on

degree of grades’ influence on heavy trucks

Get PCE factors (ET, ER ):

Then:

where:

PC, PT, PR = Percentages of cars, trucks and recreationalvehicles in the traffic stream

R RT T C

HV

E P E P P

f

++

=%100



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f f hvhv ExampleExample

Rolling terrain, 10% trucks, 5% RVs

From the PCE table, ET=2.5, ER = 2.0

So:

Note that the HCM provides a different formula for

f HV which is algebraically equivalent:

where FT and FR are the fractions (or decimal

proportions) of trucks and RVs in the traffic stream

833.0)2)(5()5.2)(10(85

100100=

++=

++=

R RT T C

HV E P E P P

f

)1()1(11

−+−+=

R RT T

HV E F E F

f



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SFR Example (Prob. Type 2)SFR Example (Prob. Type 2) A 6-lane freeway in rolling terrain has 10% trucks,

5% RVs, FFS=70 mph & PHF=0.9. Find the max.hourly volume for LOS D?

As before, f HV=0.833; assume f p=1

From Ex. 23-2, MSFD= 2150 pcplph

So:

But this is the maximum peak 15 min. flow rate.

The corresponding max. hourly volume is:

vph pcplph f f N MSF SF p HV D D 5373)1)(833.0)(3)(2150())()()(( ===

vphvph PHF SF V D D

4836)9.0)(5373())(( ===



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LOS Example (Prob. Type 1)LOS Example (Prob. Type 1)

A 6-lane freeway in rolling terrain has 10%

trucks, 5% RVs, FFS=70 mph & PHF=0.9.The design hourly volume for the year 2020 is5800 vph. Find the corresponding LOS.

As before, f HV=0.833 and f p=1

So:

In Ex. 23-2, 2579 pcplph exceeds the idealcapacity of 2400 pcplph, so LOS = F. (For an

8-lane freeway, v = 1934 pcplph & LOS = D)

pcplphvph

f f N PHF

V v

p HV

2579)1)(833.0)(3)(9.0(

5800

))()()((===



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22--Lane, 2Lane, 2--Way Rural HighwaysWay Rural HighwaysHCM MethodologyHCM Methodology

Determines the capacity/LOS for 2-lanehighways outside the influence of signalized or significant unsignalized intersections

The analysis methodology considers eachdirection separately & both directions together

Three problem types are common:

– Given traffic volume, find LOS – Given a target LOS, find the maximum volume

– Determine the improvement to LOS from adding a

passing or climbing lane



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22--Lane 2Lane 2--Way HighwaysWay HighwaysLevel of ServiceLevel of Service

The HCM 2000 recognizes two classes of 2-lane2-way rural highways:

– Class I: Purpose is mobility, long trip lengths

– Class II: Purpose is land access, recreational drives,short trip lengths

LOS for Class I highways based on (worse case):

– Average Travel Speed (ATS)

– Percent Time Spent Following (PTSF)

LOS for Class II highways based on only Percent

Time Spent Following (PTSF)



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22--Lane (Class I) LOSLane (Class I) LOS



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Steps to Obtain Directional LOSSteps to Obtain Directional LOS Calculate equivalent 15-min. flow rate for analysis

direction & opposite direction for both ATS & PTSF:

where: vi = 15 min. flow for the direction (pcph)

Vi = hourly volume for the direction (vph)

PHF = Peak Hour Factor for the direction

f G

= grade adjustment factor for direction & crit.

f HV = heavy vehicle factor for direction & crit.

Use vd and vo to calculate ATS and PTSF

Look up the Level of Service

))()(( HV G

d d

f f PHF

V v =

))()(( HV G

oo

f f PHF

V v =



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Steps for Composite Analysis of Steps for Composite Analysis of Both DirectionsBoth Directions

Calculate equivalent 15-min. flow rate for analysis for both direction:

where: v = 15 min. flow for the direction (pcph)

V = hourly volume for the direction (vph)

PHF = Peak Hour Factor for the direction

f G = grade adjustment factor for direction & crit.

f HV = heavy vehicle factor for direction & crit.

))()(( HV G f f PHF

V v =



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EstimatingEstimating f f HVHV

Given the tabulated PCE values (ET and ER ), f HV iscalculated exactly as before:

where:

PC, PT, PR = Percentages of cars, trucks and recreationalvehicles in the traffic stream

R RT T C

HV E P E P P

f ++

=%100



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Estimating FFSEstimating FFS

As with freeways, FFS is obtained from:

– Direct measurement in the field (preferred)

– Direct measurement at a similar facility (2nd best)

– An HCM estimation equation similar to freeways

Where:

– BFFS = Base free flow speed – f LS = Adjustment for lane and shoulder width, mi/hr

– f A= Adjustment for access point density, mi/hr

A LS f f BFFS FFS −−=



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Steps to Obtain LOS (Cont.)Steps to Obtain LOS (Cont.) After calculating vd & vo, compare to capacity:

– vd ≤ 1700 pcph & vo ≤ 1700 pcph (1-way capacity) – (vd + vo) ≤ 3200 pcph (two-way capacity)

If any capacity test fails, LOS = F

Otherwise, calculate ATS and PTSF; look up LOS

Note: LOS for both directions combined is determinedin much the same way, except 2-way flow, v, replacesvd and vo, and some factor values are different. Also,2-way LOS is only defined for level & rolling terrain.

npod d d f vv FFS ATS −+−= )(00776.0

)1(100bd av

d

npd d

e BPTSF

f BPTSF PTSF

−=

+=



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Steps to Obtain LOS (Cont.)Steps to Obtain LOS (Cont.)

Developed by

Robert Layton,

Oregon St. Univ.



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Many Tabulated Factors NeededMany Tabulated Factors Needed f G and f HV values different for ATS & PTSF and for

level/rolling terrain & grades ≥ 3% and ≥ 0.6 mi. long

(tables for f G in level/rolling terrain are shown below)



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Many Tabulated Factor (#2)Many Tabulated Factor (#2) “Specific grade” factors apply if grades ≥ 3%

and ≥ 0.6 mi. long. Otherwise, the following

PCE factors apply for level & rolling terrain:



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Many Tabulated Factor (#3)Many Tabulated Factor (#3) Specific Grade f G and PCE factors for ATS and PTSF

vary with grade and length of grade. For example:

Note: Exhibits truncated



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Many Tabulated Factor (#4)Many Tabulated Factor (#4) The f np factors in equations for ATS & PTSF varywith Percent No Passing, FFS, and opposing flow:

Note: Exhibits truncated



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Many Tabulated Factor (#5)Many Tabulated Factor (#5) The “a” and “b” factors in the equation for

PTSF (BPTSF) vary with opposing flow:

Capacity and LOS for Uninterrupted Flow_IITKGP

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