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Traffic Flow Characteristics

Traffic Flow Characteristics. Recap . Spacing. Recap . Clearance. Recap . # vehicles/Distance. Density. Recap…. Speed ( v or u ) – ft/sec or mph Flow ( q ) – veh/sec or vph Density ( k ) – veh/ft or vpm Spacing ( s ) – ft/veh Headway ( h ) – sec/veh Clearance ( c ) – ft/veh

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Traffic Flow Characteristics

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  1. Traffic Flow Characteristics

  2. Recap... Spacing

  3. Recap... Clearance

  4. Recap... # vehicles/Distance Density

  5. Recap… • Speed (v or u) – ft/sec or mph • Flow (q) – veh/sec or vph • Density (k) – veh/ft or vpm • Spacing (s) – ft/veh • Headway (h) – sec/veh • Clearance (c) – ft/veh • Gap (g) – sec/veh

  6. Fundamental Relationships • q = k v (veh/hr) = (veh/mi)  (mi/hr) • h = 1 /q (sec/veh) = 1 / (veh/hr)  (3600) • s = 1 /k (ft/veh) = 1 / (veh/mi)  (5280)

  7. Two Categories of Traffic Flow (Models) 1) Uninterrupted flow • Vehicle drivers are interactive with other drivers only, without interference from other control devices (e.g. Traffic signals, crosswalk, pedestrians, etc. ) to affect the flow • freeways, multilane highways, 2 lane highways

  8. Two Categories of Traffic Flow (Models) 2) Interrupted Flow • Vehicle drivers are subjected to many other control devices, such as traffic controls, stop signs, pedestrians, etc.signalized streets, unsignalized streets with stop signs, arterials, transits, pedestrian walkways, and bicycle paths

  9. Characteristics of Uninterrupted Flow q= n/t=n/t*[(1/l)/(1/l)]= k*v q= flow in vehicles per unit time n= number of vehicles passing some designated roadway during time t t= duration of time interval l= length of roadway used for travel time measurement of vehicles v= space mean speed (mph) k= density (veh/mi)

  10. General Speed-Density Relationship S V Free normal flow forced flow Traffic Jam Q Capacity K Density at Capacity Jam Density

  11. General Speed-Density Relationship K V Traffic Jam Free forced flow normal flow Q Capacity K Density at Capacity Jam Density

  12. General Speed-Density Relationship Q V Capacity Free Traffic Jam V K Density at Capacity Jam Density

  13. Capacity • The maximum flow for the facility • When area under speed vs. density curve is at maximum • Reached when the product of increasing density (k) and decreasing speed (v) result in the maximum of vehicles passing a certain point.

  14. Congested (unstable) Flow • High density-low speed • No available gaps for entering vehicles • Any vehicles that do enter create a disturbance not easily dissipated

  15. Uncongested (stable) Flow • Low density-high speed - Plenty of gaps for merging traffic

  16. Greenshield’s Model • Assume a linear relationship between v and k: vf Low Density = High Speed High Density = Low Speed kj

  17. vf v0 Greenshield’s Model Q Max flow qmax K Kj K0

  18. Greenshield’s Model V Vf 1/k0=s0 V0 Q Qmax Max flow

  19. q, v, k – Greenshield’s Free flow (measured) v Jam Density (measured) k

  20. q, v, k – Greenshield’s Free flow (measured) Max Flow v v Jam Density (measured) q k

  21. q, v, k – Greenshield’s Max Flow q Density at Capacity k Free flow (measured) Max Flow v v Jam Density (measured) q k

  22. Capacity Same flow! q, v, k – Greenshield’s • How do you asses quality of service? • What region possesses better quality of service?

  23. 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?

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