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Traffic Assignment Part I. CE 573 Transportation Planning Lecture 16. Objectives. Define traffic assignment assumptions Mathematically define relationship between OD trips and network Load traffic onto the network. Network Loading.

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traffic assignment part i

Traffic Assignment Part I

CE 573 Transportation Planning

Lecture 16

  • Define traffic assignment assumptions
  • Mathematically define relationship between OD trips and network
  • Load traffic onto the network

Michael Dixon

network loading
Network Loading

The basic objective is to assign traffic in a reasonable fashion that approximates, on the aggregate scale, how traffic uses the transportation network.

  • Assign traffic (vehicle trips) to the links
  • Approximates traffic use of network
  • Assumptions:
    • driver’s informationperfectly informed
    • driver response to informationperception of cost
    • driver objectivesminimize cost
  • Traffic assignment resultUser Equilibrium
    • no driver can reduce their travel costs from i to j by changing routes

Michael Dixon

Zone A

Zone B

Michael Dixon

basic inputs to traffic assignment network loading
Basic Inputs to Traffic Assignment (network loading)
  • Trip matrixconvert from person trips to vehicle trips By trip purpose
    • HBW: 1.1 person trips/veh trip
    • HBO: 1.6 person trips/veh trip
  • Network components
    • Links
    • centroid connectors
    • nodes
    • link travel costs
  • Route selection criteria/rules
    • Cost function
    • Minimize cost

Michael Dixon

route selection criteria rules
Route Selection Criteria/Rules
  • Routing concerns
    • stochasticdifference in motorist perceptions (quality of information and sensitivities to costs)
    • congestedcapacity constrained
  • Classification scheme for traffic assignment algorithms

Michael Dixon

basic steps of traffic assignment methods
Basic Steps of Traffic Assignment Methods
  • Identify routes
    • stored in tree
    • output from tree building algorithm
  • Assign trip matrix
    • to routes
    • creates flows on links
  • Check for convergence to user equilibrium

Michael Dixon

assigning the trip matrix to routes
Assigning the Trip Matrix to Routes
  • Use Dijkstra’s algorithm to build the minimum cost path trees
    • Have min cost path tree for all origins
    • Let’s use a link index to represent these path trees
    • a  index for each link
    • i  index for the origin zone
    • j  index for the destination zone
  • Let’s put all of the link indices () in matrix form, link choice matrix (P)
    • One dimension is O-D pairs
    • Another dimension is links
  • Now cumulatively assign all of the O-D pair volumes to their respective shortest path links

Michael Dixon

assigning o d pair volumes
Assigning O-D Pair Volumes
  • Cumulatively to their respective shortest path links
  • This is called All-or-Nothing Assignment
    • no representation of traffic effects on travel costs
    • Only one path per O-D pair
    • Just like our link choice matrix

Michael Dixon

assigning o d pair volumes2
Assigning O-D Pair Volumes
  • Assume a vehicle occupancy of
    • 1 person trips/veh trip

Michael Dixon

link travel costs
Link Travel Costs
  • Until now, constant link costs.
  • Link costs should be f(traffic volume).
  • Need a link cost function.
  • BPR function

Michael Dixon