Lecture 5: Traffic Distribution and Assignment in Impact Analyses

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Highway Traffic and Safety Analyses
Lecture 5 Traffic Distribution and Assignment in
Impact Analyses
Purdue University School of Civil
Engineering West Lafayette
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Lecture Outline

• Four-step traffic network modeling (large
  projects)
• Traffic forecast task for small and medium
  projects
• Trip distribution
• Analogy method
• Crude gravity method
• Traffic assignment to routes
• Non-car trips (pedestrians, bikes, public transit)

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Four-Step Traffic Network Modeling (Large
Projects)
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Traffic Forecast Task for Medium and Small
Projects

• Background traffic (car and non-car)
• Trends or growth factor methods
• Traffic already assigned
• New developments (analyzed one and other)
• Trip generation tables
• Car trips only
• How to distribute, and assign car traffic
  generated by new developments?
• What about non-car trips?

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Traffic Forecast Task for Medium and Small
Projects
Impact Area (Study Area)
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Traffic Distribution

• Judgment (most popular)
• Analogy method
• Crude gravity method

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Analogy Method

• Existing similar development
• Near the proposed development
• Data collection techniques
• License plate technique
• Driver response survey

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Market Area
More than 80 of trips
System Area
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Market Area
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Market Area
Impact Area
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Gravity Method
where Tij number of trip from zone i to zone
j, Pi trip production in zone i, Aj trip
attraction in zone j, Fij inverse function of
travel time from i to j, Kij adjustment factors
for flow Tij.
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Crude Gravity Method

• whereTij trips between zones i and j,
• P0 outbound trips from the new development,
• A0 inbound trips to the new development,
• Sj relevant socioeconomic characteristic of
  zone j in the market area.

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Crude Gravity Method
Impact Area
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Crude Gravity Method
2,000 outbound trips/day
3,000 population no work places
6,000 population 1,000 work places
1,000 population no work places
Impact Area
10,000 population 5,000 work places
Residential development
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Crude Gravity Method
2,000 outbound trips/day
3,000 population no work places
6,000 population 1,000 work places
1,000 population no work places
Impact Area
10,000 population 5,000 work places
Office building
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TripDistribution(daily)
50
35
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Route Assignment
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Route Assignment
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Route Assignment

• Judgment (most popular)
• Simplified logit method

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Trip AssignmentLogit Method

• Utility of route U function of route length,
  travel time, trip cost, etc.
• Example
• U - 0.5 x travel time in minutes (just an
  example)
• Two routes, L1 1.5 mi, V1 30 mi/h,
• L2 2.0 mi, V2 35 mi/h.
• U1 -1.50, U2 -1.71

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Trip AssignmentLogit Method

• Logit equation

• where Pi proportion of travelers using route
  i,Ui utility of route i.
• Example
• U1 -1.50, U2 -1.71,

• In the all-or-nothing method, all the trips are
  assigned to the best route.

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Example Traffic Forecast(format important!)
230 traffic generated by the development (500)
total traffic
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Non-car Trips

• Judgment
• Analogy method
• Logit method