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Maximizing Highway Capacity for Efficient Traffic Flow Analysis

Learn how to analyze freeway capacity to optimize traffic flow using level of service (LOS) concepts. Understand peak hour factors and determine the number of lanes needed for a desired LOS. Detailed procedures and examples provided.

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Maximizing Highway Capacity for Efficient Traffic Flow 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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