Traffic Flow Characteristics

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Traffic Flow Characteristics
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Recap...
Spacing
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Recap...
Clearance
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Recap...
vehicles/Distance
Density
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Recap

• Speed (v or u) 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

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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)

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Two Categories of Traffic Flow (Models)

• 1) Uninterrupted flow
• Vehicle drivers are interactive with other
  drivers only, without interference from other
  control devices (e.g. Traffic signals, crosswalk,
  pedestrians, etc. ) to affect the flow
• freeways, multilane highways, 2 lane highways

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Two Categories of Traffic Flow (Models)

• 2) Interrupted Flow
• Vehicle drivers are subjected to many other
  control devices, such as traffic controls, stop
  signs, pedestrians, etc.signalized streets,
  unsignalized streets with stop signs, arterials,
  transits, pedestrian walkways, and bicycle paths

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Characteristics of Uninterrupted Flow

• q n/tn/t(1/l)/(1/l) kv
• q flow in vehicles per unit time
• n number of vehicles passing some designated
  roadway during time t
• t duration of time interval
• l length of roadway used for travel time
  measurement of vehicles
• v space mean speed (mph)
• k density (veh/mi)

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General Speed-Density Relationship
S
V
Free
normal flow
forced flow
Traffic Jam
Q
Capacity
K
Jam Density
Density at Capacity
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General Speed-Density Relationship
K
V
Traffic Jam
Free
forced flow
normal flow
Q
Capacity
K
Jam Density
Density at Capacity
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General Speed-Density Relationship
Q
V
Capacity
Free
Traffic Jam
V
K
Jam Density
Density at Capacity
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Capacity

• The maximum flow for the facility
• When area under speed vs. density curve is at
  maximum
• Reached when the product of increasing density
  (k) and decreasing speed (v) result in the
  maximum of vehicles passing a certain point.

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Congested (unstable) Flow

• High density-low speed
• No available gaps for entering vehicles
• Any vehicles that do enter create a disturbance
  not easily dissipated

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Uncongested (stable) Flow

• Low density-high speed
• - Plenty of gaps for merging traffic

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Greenshields Model

• Assume a linear relationship between v and k

vf
Low Density High Speed
High Density Low Speed
kj
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Greenshields Model
Q
Max flow
qmax
K
Kj
K0
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Greenshields Model
V
Vf
1/k0s0
V0
Q
Qmax
Max flow
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q, v, k Greenshields
Free flow (measured)
v
Jam Density (measured)
k
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q, v, k Greenshields
Free flow (measured)
Max Flow
v
v
Jam Density (measured)
q
k
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q, v, k Greenshields
Max Flow
q
Density at Capacity
k
Free flow (measured)
Max Flow
v
v
Jam Density (measured)
q
k
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q, v, k Greenshields

• How do you asses quality of service?
• What region possesses better quality of service?

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Example

• I-5 over the ship canal bridge has 4 lanes in
  each direction. The northbound capacity is 8200
  veh/hr/lane and the free-flow speed is 65 mph.
  What is the maximum flow rate, maximum density,
  jam density? If a one-hour vehicle count in the
  northbound direction for the outside lane gives
  7034 vehicles in a non-congested condition, what
  is the estimated space mean speed of these 7034
  vehicles?