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# Traffic Assignment Part I - PowerPoint PPT Presentation

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

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

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 B

Michael Dixon

• 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

• centroid connectors

• nodes

• Route selection criteria/rules

• Cost function

• Minimize cost

Michael Dixon

• 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

• Identify routes

• stored in tree

• output from tree building algorithm

• Assign trip matrix

• to routes

• Check for convergence to user equilibrium

Michael Dixon

• 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

• Now cumulatively assign all of the O-D pair volumes to their respective shortest path links

Michael Dixon

Michael Dixon

Michael Dixon

Michael Dixon

• 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

Michael Dixon

• Assume a vehicle occupancy of

• 1 person trips/veh trip

Michael Dixon

• Until now, constant link costs.

• Link costs should be f(traffic volume).

• Need a link cost function.

• BPR function

Michael Dixon