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Traffic Concepts

Traffic Concepts. CEE 320 Anne Goodchild. Outline. Basic Concepts Flow Rate Spacing Headway Speed Density Relationships Example. Concepts. Definitions Volume, Speed, Density relationships Speed Space mean speed Time mean speed. Flow Rate (q) .

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Traffic Concepts

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  1. Traffic Concepts CEE 320Anne Goodchild

  2. Outline • Basic Concepts • Flow Rate • Spacing • Headway • Speed • Density • Relationships • Example

  3. Concepts • Definitions • Volume, Speed, Density relationships • Speed • Space mean speed • Time mean speed

  4. Flow Rate (q) • The number of vehicles (n) passing some designated roadway point in a given time interval (t) • Units are typically vehicles/hour • Flow rate is different than volume

  5. Spacing • The distance (ft) between successive vehicles in a traffic stream, as measured from front bumper to front bumper

  6. Headway (h) • The time (in seconds) between successive vehicles, as their front bumpers pass a given point.

  7. Headway From HCM 2000

  8. Speed • Time mean speed (spot speed) • Arithmetic mean of all instantaneous vehicle speeds at a given “spot” on a roadway section • Space mean speed (u) • The mean travel speed of vehicles traversing a roadway segment of a known distance (d) • More useful for traffic applications

  9. Space Mean Speed • It is the harmonic mean (1/H = 1/a + 1/b + …) • Space mean speed is always less than time mean speed

  10. Time Mean vs. Space Mean Speed From HCM 2000

  11. The number of vehicles (n) occupying a given length (l) of a lane or roadway at a particular instant Unit of density is vehicles per mile (vpm). Density (k)

  12. Other Concepts • Free-flow speed (uf) • Jam density (kj) • Capacity (qm)

  13. Speed vs. Density ufFree Flow Speed Speed (mph) kjJam Density Density (veh/mile)

  14. Flow vs. Density Congested Flow Optimal flow, capacity, qm FLow (veh/hr) kmOptimal density kjJam Density Uncongested Flow Density (veh/mile)

  15. Speed vs. Flow ufFree Flow Speed Uncongested Flow um Speed (mph) Optimal flow, capacity, qm Congested Flow Flow (veh/hr)

  16. 3-D Model

  17. Example I-5 over the ship canal bridge has 4 lanes in each direction. The northbound capacity is 8200 veh/hr/lane and the free-flow speed is 65 mph. What is the maximum flow rate, maximum density, jam density? If a one-hour vehicle count in the northbound direction for the outside lane gives 7034 vehicles in a non-congested condition, what is the estimated space mean speed of these 7034 vehicles?

  18. Traffic – Time of Day Patterns

  19. From WSDOT 2003 Annual Traffic Report

  20. From WSDOT 2003 Annual Traffic Report

  21. Traffic Profile SR 520 General Purpose Lanes 1997 Weekday Average West bound Ea st bound NE 60th St. 148 th Ave. 405 B’vue. Way 92nd Ave. 84th Ave. 520 Lk. Wash. Arboretum Montlake Blvd. 5 0 2 4 6 8 10 12 2 4 6 8 10 12 0 2 4 6 8 10 12 2 4 6 8 10 12 AM PM AM PM Time Time Uncongested, near Restricted movement More heavily congested, Extremely congested, speed limit but near speed limit 45 - 55 mph unstable flow

  22. Primary References • Mannering, F.L.; Kilareski, W.P. and Washburn, S.S. (2005). Principles of Highway Engineering and Traffic Analysis, Third Edition. Chapter 5 • Transportation Research Board. (2000). Highway Capacity Manual 2000. National Research Council, Washington, D.C.

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