Traffic Assignment for the Wiscasset Route 1 Corridor Study

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Traffic Assignment for theWiscasset Route 1
Corridor Study

• Feb 7, 2008

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Traffic Assignment

• A tool to estimate the amount of traffic on a
  network if changes are made to the network
• Needs
• A network of links and nodes (in a study area)
• A trip table (with origins and destinations)

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Network

• Links segments of the network
• Links have length, speed, and capacity
• Nodes where segments meet
• Special nodes called stations and zones are where
  trips begin or end

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Network
C
B
E
D
A
F
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Trip Table
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Traffic Assignment Methods

• All-or-Nothing
• Capacity Restraint
• Incremental
• Equilibrium

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All-or-Nothing

• Simple
• All trips between an origin and destination are
  assigned to (or loaded onto) the same route (the
  shortest route when there is no congestion)
• Often is less realistic than other methods

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Network
C
B
E
D
A
All-or-Nothing Assignment Works Well
F
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Network
C
B
?
?
E
D
A
All-or-Nothing Assignment May Not Work Well
F
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Principles of Capacity Restraint

• Every link has a capacity
• As traffic is loaded on a link uses up the links
  capacity, speed decreases and travel time
  increases
• As travel time on a route increases, other routes
  may become shorter travel time routes
• Many travelers prefer the shortest travel time

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Incremental Capacity Restraint

• Trip table is loaded onto the network in
  increments
• The first increment is like an all-or-nothing
  assignment
• Speeds and travel times are adjusted for each
  link, based on the number of trips loaded on that
  link
• Additional increments are assigned based on the
  adjusted speeds and travel times

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Network
C
B
2 3
1 4
E
D
A
Incremental Assignment
F
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Equilibrium Capacity Restraint

• Speeds and travel times are adjusted for each
  link, based on the number of trips loaded on that
  link
• Trips are loaded in a balanced manner so that
  competing routes have the same travel time
  (equilibrium)
• Equilibrium can be reached even if some travelers
  do not consider travel time

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Network
C
B
Travel time Travel time
E
D
A
Equilibrium Assignment
F
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Equilibrium AssignmentChosen for
Wiscasset-Edgecomb

• Exercise with P, R, T, and V

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Time Periods for Traffic Assignments

• Daily
• statewide model
• Gateway1 model
• Peak-Hour
• PACTS model

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Time Periods for Wiscasset-EdgecombTraffic
Assignments

• Hourly for each hour of the day

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Route Signing Route Choice

• Route signing can cause traffic to choose a
  longer travel time route
• Route signing is more effective with travelers
  unfamiliar with the network
• As travelers become more familiar with the
  network, some will change to a shorter route
• The effects of route signing may be temporary

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Traffic Calming Route Choice

• Traffic calming on a route increases travel time
• The increased travel time will cause some
  travelers to choose a different route
• Traffic calming has a permanent effect on route
  choice