Traffic assignment part i
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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

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Traffic assignment part i

Traffic Assignment Part I

CE 573 Transportation Planning

Lecture 16


Objectives

Objectives

  • 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


Traffic assignment part i

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


Dijkstra s algorithm link indices and creating p

Dijkstra’s Algorithm, Link Indices, and Creating P

Michael Dixon


Dijkstra s algorithm link indices and creating p1

Dijkstra’s Algorithm, Link Indices, and Creating P

Michael Dixon


Dijkstra s algorithm link indices and creating p2

Dijkstra’s Algorithm, Link Indices, and Creating P

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 volumes1

Assigning O-D Pair Volumes

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


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