# Traffic Flow Characteristics (2) - PowerPoint PPT Presentation

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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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Traffic Flow Characteristics (2)

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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

Spacing

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

Q

Max flow

qmax

K

Kj

K0

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