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Freeway Capacity Analysis

Freeway Capacity Analysis. CE331 Transportation Engineering. 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. Some Terms. Capacity

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Freeway Capacity Analysis

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  1. Freeway Capacity Analysis CE331 Transportation Engineering

  2. 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

  3. Some Terms • Capacity • Max 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 conditions • Level of congestion • Mostly determined by density

  4. Level of Service (LOS)

  5. Level of Service A

  6. Level of Service F

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

  8. Weaving area Basic freeway segment Ramp junction Freeway Components • Basic freeway sections • Weaving areas • Ramps and ramp junctions

  9. “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)

  10. Procedure for Determining LOS • Estimate free flow speed (FFS) • Calculate equivalent flow rate (vp) • Compute density (D=vp/S) and compare D to values in Exhibit 23-3 to determine LOS

  11. Step 1:Free Flow Speed (FFS) Adjust for existing conditions FFS = BFFS - fLW - fLC - fN – fID • BFFS – base FFS • Rural: 75mph • Urban: 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

  12. Adjust for vehicle mix and users Step 2:Equivalent Flow Rate (vp) V: directional hourly volume (vph) PHF: peak hour factor N: number of lanes in one direction fHV: heavy vehicle adj. factor fp: driver population adj. factor, mostly =1

  13. Peak Hour Factor (PHF) • Describes concentration of traffic within one hour • Definition: • V – hourly volume • V15 – maximum 15-min flow rate within the hour

  14. Max Flow Volume PHF Example PHF = 2150/2300 = 0.94

  15. Heavy Vehicle Adj. Factor (fHV) 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

  16. Step 3:Calculate Density and Compare • Density (D) = vp / S • Compare D with threshold values in Exhibit 23-3 and determine LOS

  17. Types of Analysis • Operational Determine LOS • Design & Planning Determine number of lanes

  18. 1.9 75 1.2 0.0 0.0 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? = 71.9 mph FFS = BFFS - fLW - fLC - fN - fID

  19. 2100 0.0 1.5 V vp = PHF N fhv fp 1.00 0.90 2 0.06 Example 1 (cont’d) = 1202 pcphpl = 0.97 fhv = 1/[1+PT(ET-1)+PR(ER-1)]

  20. Example 1 (cont’d) D = vP/S = 1202/71.9 = 16.72 vpm

  21. 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.

  22. 0.0 0.0 4.5 0.0 70 (urban) Example 2 (2/7) FFS = BFFS - fLW - fLC - fN - fID = 65.5 mph

  23. 3200 2.5 1.5 V vp = PHF N fhv fp 1.00 0.90 2 0.05 0.05 Example 2 (3/7) = 1956 pcphpl fhv = 1/[1+PT(ET-1)+PR(ER-1)] = 0.91

  24. Example 2 (4/7) D = vP/FFS = 1956/65.5 = 31.2 vpm Y LOS D

  25. V vp = 0.0 0.0 3.0 70 0.0 PHF N fhv fp 1.00 3200 0.90 3 0.91 Example 2 (5/7) FFS = BFFS - fLC - fLW - fN - fID = 67.0 mph = 1304 pcphpl

  26. Example 2 (6/7) D = vP/FFS = 1304/67 = 19.5 vpm Y LOS C

  27. pcphpl mph Example 2 (7/7) D = vP/FFS = 1304/67 = 19.5 vpm Y LOS C Use Graph or LOS Table: Source: HCM 2000, Ch. 23, p.23-3

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