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Traffic Flow Characteristics (2) PowerPoint Presentation

Traffic Flow Characteristics (2)

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Traffic Flow Characteristics (2) . Traffic Flow Characteristics (2) . Traffic Flow Characteristics (2) . Learning Objectives. To differentiate between interrupted and uninterrupted flow facilities To define general and linear speed-density relationships

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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

Recap

Spacing

Recap

Clearance

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!

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)

Types of Facilities

- Uninterrupted flow
- Freeways
- Multilane highways
- Two-lane highways

Types of Facilities

- Interrupted flow
- Signalized streets
- Un-signalized streets with stop signs
- Transit lanes
- Pedestrian walkways

General Speed-Density Relationship

p.130

S

V

Free

normal flow

forced flow

Traffic

Jam

Q

Capacity

K

Density at Capacity

Jam Density

General Speed-Density Relationship

p.137

K

V

Traffic

Jam

Free

forced flow

normal flow

Q

Capacity

K

Density at Capacity

Jam Density

General Speed-Density Relationship

p.137

Q

V

Capacity

Free

Traffic

Jam

V

K

Density at Capacity

Jam Density

Greenshield’s Model

- Assume a linear relationship between v and k:

Low Density = High Speed

vf

High Density = Low Speed

kj

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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