Traffic Flow Characteristics (2)

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# Traffic Flow Characteristics 2 - PowerPoint PPT Presentation

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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Presentation Transcript

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
• To derive, sketch, and apply Greenshield’s Model of traffic flow
Recap

Spacing

Recap

Clearance

Recap

# vehicles/Distance

Density

Recap…
• Speed (v) – ft/sec or mph
• Flow (q) – veh/sec or vph
• Density (k) – veh/ft or vpm
• Spacing (s) – ft/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

vf

v0

Greenshield’s Model

Q

Max flow

qmax

K

Kj

K0

Greenshield’s Model

V

Vf

1/k0=s0

V0

Q

Qmax

Max flow

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