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Course Logistics
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  1. Course Logistics • Course grading scheme correct • Team assignments posted • HW 1 posted • Note-taker needed • Website and Transportation wiki • Traffic Concepts

  2. Traffic questions • Have you ever driven out of a traffic jam only to find that nothing was apparently causing it? • Why does stop and go traffic happen? • Why does traffic slow down as it gets heavier? • How do we predict congestion?

  3. Traffic Concepts CEE 320Anne Goodchild

  4. Traffic – Time of Day Patterns

  5. How can we describe traffic?

  6. How can we describe traffic? • Consider one lane of traffic • Traffic is constantly changing

  7. From WSDOT 2003 Annual Traffic Report

  8. From WSDOT 2003 Annual Traffic Report

  9. How would you model traffic flow? • As a liquid? • As a glacier? • Discrete event simulation? • …

  10. Concepts • Flow • Speed • Density

  11. Flow (q) • The number of vehicles (n) passing some designated roadway point in a given time interval (t) • Units typically vehicles/hour • Volume typically refers to flow in an hour

  12. Flow • Text also uses flow rate • Flow is constantly varying! • Analysis flow rate is peak 15-minute flow within the hour of interest.

  13. Spacing

  14. How much space do you leave between vehicles? • 2 chevrons! • Depends on speed • Varies for each person

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

  16. Spacing • Density • the number of vehicles over a length of freeway • Occupancy • Measured by loop detectors • The percent of time the loop is covered by a vehicle

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

  18. Headway From HCM 2000

  19. How do we measure average speed? • The average speed of vehicles that pass by a specific point in space over a specific time period (TMS). • Time necessary for a vehicle to travel a length of roadway (SMS).

  20. Measuring Speed • Time mean speed • Taken at a specific point • Average of instantaneous speeds • Space mean speed (u) • Harmonic speed • Look at a segment of roadway • Average speed of all vehicles in that segment

  21. Time Mean Speed • Arithmetic mean of speeds observed at some point • Easy to measure radar

  22. Space Mean Speed • It is the harmonic mean • Used in traffic models, but harder to measure

  23. Example • You are in a vehicle traveling a total of 10 miles. • the first 5 miles you travel at 40 mph • the next 5 miles you travel at exactly 60 mph • What is your average speed over the time you spent traveling that 10 miles? • What is your average speed over that distance?

  24. Average speed over time and average speed over distance are different 10 miles 40 mph 60 mph 5 minutes 7.5 minutes 12.5 minutes 7.5 minutes 5 minutes

  25. Example time • You are in a vehicle traveling a total of 10 miles. • the first 5 miles you travel at 40 mph • the next 5 miles you travel at exactly 60 mph • What is your average speed over the time you spent traveling that 10 miles? • 5 miles at 40 mph = 7.5 minutes • 5 miles at 60 mph = 5 minutes • weighted average = (40(7.5) + 60(5))/(7.5 + 5) = 48 mph

  26. Example - distance • 5 vehicles over a given 1 mile section take 1.0, 1.2, 1.5, 0.75 and 1.0 minutes respectively • Average travel time • 5.45/5 = 1.09 minutes = 0.0182 hours • Therefore, average speed over that distance 1 mile/0.0182 hours = 55.05 mph

  27. 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 - konzentration)

  28. Number of vehicles in length of segment Inverse of average spacing Density (k)

  29. Density

  30. A model for the relationship between flow, density, and speed Represents idealized behavior and fundamental relationships Useful for traffic analysis Traffic Flow Theory

  31. Complete the charts speed flow density density

  32. Complete the charts speed flow

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

  34. Additional definitions • Free-flow speed (uf) • The speed at which vehicles will travel unimpeded • Jam density (kj) • The density of vehicles in stopped traffic • Capacity (qm) • The maximum flow a section of roadway can maintain

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

  36. Speed vs. Flow ufFree Flow Speed Uncongested Flow um Speed (mph) Highest flow, capacity, qm Congested Flow Flow (veh/hr) qm is bottleneck discharge rate

  37. Measurement • Density can easily be measured by remote sensing, but has historically been difficult to measure • Use occupancy obtained from loop-detectors • TMS more easy to measure than SMS • Use correction or approximation • Easy to measure with remote sensing (GPS) • Flow is easy to measure • Occupancy is measure of density • Only need to measure 2 of 3

  38. Loop Detector Freeway Monitoring

  39. Loop Detector Signatures

  40. Inductance Loop Detectors Loop inductance decreases when a car is on top of it. Inductance T = ton 0 ton T = toff toff Time

  41. Inductance High low Tn tn1 tn2 tn3 Tn+1 Time Inductance Loop Detectors • Single loops can measure: • Occupancy (O): % of time loop is occupied per interval • Volume (N): vehicles per interval Single loop measurements?

  42. How do you estimate speed from a single loop detector? • You know how long it took the object to pass over the detector • You know the percent of time (in a short interval) the loop was covered by a vehicle • What else do you need to know?

  43. Can We Get Speed from a Single Loop? EVL ~ vehicle length + detector length (24 feet)

  44. Estimating Speed from a Single Loop? • Using typical traffic data 100 converts percent to decimal

  45. What speed is this? • TMS