Freeway Capacity Analysis

CE331 Transportation Engineering

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Objectives

Understand the concept of level of service (LOS)

Define peak hour factor (PHF) Determine the LOS for a given

freeway Determine the number of lanes to

achieve a given LOS

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Some Terms

CapacityMax hourly flow rate for a roadway

segment under existing conditions Free Flow Speed (FFS)

Speed of traffic when no vehicles are present

In practice, measured at flow < 1,300 pcphpl Level of Service (LOS)

Qualitative measure of traffic conditionsLevel of congestionMostly determined by density

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Level of Service (LOS)

LOS Density (pc/mi/ln)

A 0-11

B 12-18

C 19-26

D 27-35

E 36-45

F > 45

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Level of Service A

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Level of Service F

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Freeways

Limited access roads with high design standards, divided, and at least two lanes per direction

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Freeway Components

Basic freeway sections Weaving areas Ramps and ramp junctions

Weaving area Basic freeway segment Ramp junction

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“Ideal” Conditions

12-foot lanes 6-foot right shoulder 2-foot left shoulder Only passenger cars Leveled terrain Interchange density 0.5/mi (or 2-mile spacing) Everyday commuters Five or more lanes per direction (urban only)

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Procedure for Determining LOS

1. Estimate free flow speed (FFS)2. Calculate equivalent flow rate (vp)3. Compute density (D=vp/S) and

compare D to values in Exhibit 23-3 to determine LOS

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Step 1:Free Flow Speed (FFS)

Adjust for existing conditions

FFS = BFFS - fLW - fLC - fN – fID BFFS – base FFS

Rural: 75mphUrban: 70mph

fLW – lane width adj. factor, Exhibit 23-4 fLC – lateral clearance adj. factor, Exhibit 23-5 fN – number of lanes adj. factor, Exhibit 23-6 fID – interchange density adj. factor, Exhibit 23-7

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Step 2:Equivalent Flow Rate (vp)

Adjust for vehicle mix and users

pHVp ffNPHF

Vv

V: directional hourly volume (vph)PHF: peak hour factorN: number of lanes in one directionfHV: heavy vehicle adj. factorfp: driver population adj. factor, mostly =1

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Peak Hour Factor (PHF)

Describes concentration of traffic within one hour

Definition:

V – hourly volumeV15 – maximum 15-min flow rate within

the hour

415V

VPHF

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PHF Example

Time Count

9:00-9:15 500

9:15-9:30 550

9:30-9:45 525

9:45-10:00 575

Total 2150

Volume

Hourly Flow

2000

2200

2100

2300Max Flow

PHF = 2150/2300 = 0.94

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Heavy Vehicle Adj. Factor (fHV)

)1()1(1

1

RRTTHV EPEPf

PT – truck and bus percentage

PR – recreational vehicle percentage

ET – passenger car equivalent for trucks and buses, Exhibits 23-8, 23-9, 23-11

ER – passenger car equivalent for recreational vehicles, Exhibit 23-8, 23-10

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Step 3:Calculate Density and Compare

Density (D) = vp / S Compare D with threshold values in

Exhibit 23-3 and determine LOS

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Types of Analysis

Operational

Design & Planning

Determine LOS

Determine number of

lanes

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Example 1

4-Lane rural freeway; V=2,100 vph; 6% trucks; 11-foot lanes; 4-foot right and left shoulders; 3% grade; 0.55 miles; 0.25 interchanges/mile; PHF 0.9; everyday commuters. LOS?

FFS = BFFS - fLW - fLC - fN - fID

1.975

= 71.9 mph

1.2 0.0 0.0

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Example 1 (cont’d)

vp =V

PHF N fhv fp

2100

0.90 2

fhv = 1/[1+PT(ET-1)+PR(ER-1)]

0.06

= 0.97

0.0

1.00

= 1202 pcphpl

1.5

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Example 1 (cont’d)

D = vP/S = 1202/71.9 = 16.72 vpm

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Example 2

An urban freeway segment of 1.5 miles is expected to be designed under ideal conditions and it will be located on a 3% upgrade. The directional design hourly volume is expected to be 3,200vph and it will consist of 5% trucks, 5% recreational vehicles, and 90% passenger cars. Most drivers will be everyday commuters. If similar freeways showed a PHF=0.90 and no interchanges are planned within this section, determine the number of lanes required to accommodate the traffic at minimum LOS C.

Example 2(2/7)

FFS = BFFS - fLW - fLC - fN - fID

0.0 0.0 4.570 (urban) 0.0

= 65.5 mph

Example 2 (3/7)

vp =V

PHF N fhv fp

3200

0.90 2

fhv = 1/[1+PT(ET-1)+PR(ER-1)]

0.05 2.5

= 0.91

0.05

1.00

= 1956 pcphpl

1.5

Example 2 (4/7)

D = vP/FFS = 1956/65.5 = 31.2 vpm LOS D

Example 2 (5/7)

FFS = BFFS - fLC - fLW - fN - fID

0.0 0.0 3.070 0.0

= 67.0 mph

vp =V

PHF N fhv fp

3200

0.90 3

1.00

= 1304 pcphpl

0.91

Example 2

(6/7)

D = vP/FFS = 1304/67 = 19.5 vpm LOS C

Example 2

(7/7)

pcphplmph

Use Graph or LOS Table:

Source: HCM 2000, Ch. 23, p.23-3

D = vP/FFS = 1304/67 = 19.5 vpm LOS C