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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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slide1

Traffic Flow Characteristics (2)

Traffic Flow Characteristics (2)

Traffic Flow Characteristics (2)

learning objectives
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
Recap

Spacing

recap4
Recap

Clearance

recap5
Recap

# vehicles/Distance

Density

recap6
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
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
Types of Facilities
  • Uninterrupted flow
    • Freeways
    • Multilane highways
    • Two-lane highways
types of facilities9
Types of Facilities
  • Interrupted flow
    • Signalized streets
    • Un-signalized streets with stop signs
    • Transit lanes
    • Pedestrian walkways
general speed density relationship
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 relationship11
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 relationship12
General Speed-Density Relationship

p.137

Q

V

Capacity

Free

Traffic

Jam

V

K

Density at Capacity

Jam Density

greenshield s model
Greenshield’s Model
  • Assume a linear relationship between v and k:

Low Density = High Speed

vf

High Density = Low Speed

kj

greenshield s model14

vf

v0

Greenshield’s Model

Q

Max flow

qmax

K

Kj

K0

greenshield s model15
Greenshield’s Model

V

Vf

1/k0=s0

V0

Q

Qmax

Max flow

example
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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