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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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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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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 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 (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 (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