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Urban Transportation Modeling: Traffic Flow Forecasting and Assignment Techniques

Explore traffic assignment techniques, modal split analysis, and equilibrium models in urban transportation planning. Understand user equilibrium and the impacts of route choice on vehicular flow.

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Urban Transportation Modeling: Traffic Flow Forecasting and Assignment Techniques

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  1. Travel Demand Forecasting:Traffic Assignment CE331 Transportation Engineering

  2. Land Use and Socio-economic Projections Trip Generation Trip Distribution Transportation System Specifications Modal Split Traffic Assignment Direct User Impacts Overall Procedure

  3. Traffic Assignment • What route to choose? • Mostly for auto mode • Intermodal network • Input • OD matrix by mode • Roadway network (for auto) and/or transit network • Characteristics (travel time, cost, …) of each route connecting an OD pair • Output • Vehicular flow on each link of the network

  4. Assignment Techniques • Basic Assumption • Travelers always choose the least cost route connecting the OD • All or nothing assignment • Assign all trips/vehicles to the route with minimum cost • Assume travel time does not change with volume • Equilibrium assignment

  5. 600 All or Nothing Assignment 10 min 600 veh O D 12 min 0 veh If there are 600 vehicles traveling between O and D, all of them will be assigned to the shortest route (with 10 min travel time).

  6. Problems of “All or Nothing” Assignment • Ignore capacity constraint • Ignore the fact that travel time is related to flow level

  7. Highway Performance Function • Flow-dependent travel time • Free flow travel time (tf) • Typical form:a.k.a. BPR function

  8. Equilibrium Assignment • Basic Assumptions • Travelers know the exact cost of choosing each route • Travelers always choose the routes that minimize their own travel cost

  9. User Equilibrium • Model travelers’ route choice behavior • Equilibrium condition: • Between an OD pair, • All used routes will have the same travel time; • All unused routes will have longer travel time(s).

  10. O 4500 D Example 1 t1 = 6 + 4x1 x1 (thousand vehs) t2 = 4 + x22 x2 (thousand vehs)  6 + 4x1 = 4 + x22 At equilibrium, let t1 = t2 x1 = 1.6 x2 = 2.9 x1 + x2= 4.5

  11. Population, Household by TAZ – Woodford County

  12. Estimated Speed

  13. Congestion Level (V/C ratio) Estimation

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