Traffic Flow Analysis II Lecture 13 CE 2710 Norman Garrick
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Norman W. Garrick Traffic Flow Analysis II Lecture 13 CE 2710 Norman Garrick
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Traffic Flow Analysis II Lecture 13 CE 2710 Norman Garrick. Representing Traffic Flow. The fundamental equation of traffic stream flow is q = uk - PowerPoint PPT Presentation
Transcript of Traffic Flow Analysis II Lecture 13 CE 2710 Norman Garrick
Norman W. Garrick
Traffic Flow Analysis IILecture 13CE 2710
Norman Garrick
Norman W. Garrick
Representing Traffic Flow
The fundamental equation of traffic stream flow is
q = uk
This equation is used in conjunction with the Traffic Flow Curves (next slide) in an attempt to characterize traffic flow on a macroscopic level.
However, it should be understood that these analytical representation of traffic flow does fully depicting real life traffic which is a complex and non-linear phenomena.
The complexity derive from the fact that we have a large number of vehicles interacting in a way that does not follow the laws of mechanics due to the reaction of the HUMAN driver
Norman W. Garrick
Traffic Flow Curves
u
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Norman W. Garrick
Traffic Flow Curves
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Norman W. Garrick
Traffic Flow Curves
u
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?
Norman W. Garrick
Traffic Flow Curves
u
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u
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Norman W. Garrick
Traffic Flow Curves
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Norman W. Garrick
Traffic Flow Curves
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One important point to note,
FLOW increase with SPEED but only up to a point
In other words, higher SPEED does not necessarily mean high FLOW
Why? What is the physical phenomenon that cause this?
Norman W. Garrick
Traffic Flow Curves
u
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Norman W. Garrick
Traffic Flow Curves
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Maximum FLOW occurs at medium CONCENTRATION
Norman W. Garrick
What is the Maximum Theoretical Flow on a Highway?
If we have CONCENTRATION and SPEED under different conditionswe could estimate MAXIMUM FLOW
But the relationship between SPACE (or concentration) and SPEED depends on how the average person drive.
This characterization of how people drive is some times refer to as the ‘safety regime’.
And as we have discussed before, this safety regime is hard to characterize and depends on factors such as weather, age of drivers and other cultural factors
The figure in the next slide is considered to give a SPACE/SPEED relationship that comes close to reality for American driving conditions
Norman W. Garrick
Calculating Maximum FlowSpacing versus Speed
Spa
cing
, ft
Norman W. Garrick
Calculating Maximum FlowSpeed versus Concentration (u-k)
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k = 1 / s(ave) u
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Norman W. Garrick
Calculating Maximum FlowFlow versus Speed (u-q)
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0 500 1000 1500 2000 2500Flow (q) , vehicles/ hr
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In theory qmax occurs at speed of about 35 mph
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Norman W. Garrick
Calculating Maximum FlowFlow versus Concentration (q-k)
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0 50 100 150 200 250Concentration (k), vehicles/ mile
Flow
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q = ukqmax = 2000 veh/hr
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Norman W. GarrickNorman W. Garrick
Traffic Flow CurvesMaximum Flow, Jam Concentration, Freeflow
Speed
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qqmax
qmaxkj
uf
kj - jam concentrationu = 0, k = kj
qmax - maximum flow
uf - free flow speedk = 0, u = uf
Norman W. Garrick
What is the Jam Concentration?
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Norman W. Garrick
What is the Freeflow Speed?
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