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Traffic Flow Characteristics (2) Traffic Flow Characteristics (2) Traffic Flow Characteristics (2)

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Page 1: Traffic Flow Characteristics (2). Learning Objectives To differentiate between interrupted and uninterrupted flow facilities To define general and linear

Traffic Flow Characteristics (2)

Traffic Flow Characteristics (2)

Traffic Flow Characteristics (2)

Page 2: Traffic Flow Characteristics (2). Learning Objectives To differentiate between interrupted and uninterrupted flow facilities To define general and linear

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

Page 3: Traffic Flow Characteristics (2). Learning Objectives To differentiate between interrupted and uninterrupted flow facilities To define general and linear

Recap

Spacing

Page 4: Traffic Flow Characteristics (2). Learning Objectives To differentiate between interrupted and uninterrupted flow facilities To define general and linear

Recap

Clearance

Page 5: Traffic Flow Characteristics (2). Learning Objectives To differentiate between interrupted and uninterrupted flow facilities To define general and linear

Recap

Density

# vehicles/Distance

Page 6: Traffic Flow Characteristics (2). Learning Objectives To differentiate between interrupted and uninterrupted flow facilities To define general and linear

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!

Page 7: Traffic Flow Characteristics (2). Learning Objectives To differentiate between interrupted and uninterrupted flow facilities To define general and linear

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)

Page 8: Traffic Flow Characteristics (2). Learning Objectives To differentiate between interrupted and uninterrupted flow facilities To define general and linear

Types of Facilities

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

Page 9: Traffic Flow Characteristics (2). Learning Objectives To differentiate between interrupted and uninterrupted flow facilities To define general and linear

Types of Facilities

• Interrupted flow– Signalized streets– Un-signalized

streets with stop signs

– Transit lanes– Pedestrian

walkways

Page 10: Traffic Flow Characteristics (2). Learning Objectives To differentiate between interrupted and uninterrupted flow facilities To define general and linear

General Speed-Density Relationship

Free

V

KJam

DensityDensity at Capacity

S

QTraffic

JamCapacity

normal flow

forced flow

p.130

Page 11: Traffic Flow Characteristics (2). Learning Objectives To differentiate between interrupted and uninterrupted flow facilities To define general and linear

General Speed-Density Relationship

Free

V

KJam

DensityDensity at Capacity

K

Q

Traffic Jam

Capacity

normal flow

forced flow

p.137

Page 12: Traffic Flow Characteristics (2). Learning Objectives To differentiate between interrupted and uninterrupted flow facilities To define general and linear

General Speed-Density Relationship

Free

V

KJam

DensityDensity at Capacity

Q

VTraffic Jam

Capacity

p.137

Page 13: Traffic Flow Characteristics (2). Learning Objectives To differentiate between interrupted and uninterrupted flow facilities To define general and linear

Greenshield’s Model

• Assume a linear relationship between v and k:

High Density = Low Speed

Low Density = High Speedvf

kj

kk

vvv

j

ff

Page 14: Traffic Flow Characteristics (2). Learning Objectives To differentiate between interrupted and uninterrupted flow facilities To define general and linear

Greenshield’s Model

K

QMax flow

qmax

K0Kj

vf v0 2kk

vkvq

j

ff

Page 15: Traffic Flow Characteristics (2). Learning Objectives To differentiate between interrupted and uninterrupted flow facilities To define general and linear

Greenshield’s Model

Q

V

Max flowQmax

Vf

V0

1/k0=s0

Page 16: Traffic Flow Characteristics (2). Learning Objectives To differentiate between interrupted and uninterrupted flow facilities To define general and linear

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