Traffic Flow Characteristics 2

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Traffic Flow Characteristics 2

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2. Learning Objectives To differentiate between interrupted and uninterrupted flow facilities To define general and linear speed-density relationships To derive, sketch, and apply Greenshield?s Model of traffic flow

3. Recap

4. Recap

5. Recap

6. Recap? Speed (v) ? 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 Remember, units are critical!

7. 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)

8. Types of Facilities Uninterrupted flow Freeways Multilane highways Two-lane highways

9. Types of Facilities Interrupted flow Signalized streets Un-signalized streets with stop signs Transit lanes Pedestrian walkways

10. General Speed-Density Relationship

11. General Speed-Density Relationship

12. General Speed-Density Relationship

13. Greenshield?s Model Assume a linear relationship between v and k:

14. Greenshield?s Model

15. Greenshield?s Model

16. Example Assuming a linear v-k relationship, the mean free speed is 60 mph near zero density, and the corresponding jam density is 140 vpm. Assume the average length of vehicles is 20 ft. Find: v(k) and q(k) Sketch v-k, v-q, and q-k diagrams Compute v and k at q=1000 vph Compute the average headway, spacings, clearances, and gaps when the flow is maximum


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