Travel Demand Forecasting: Traffic Assignment

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

13. Population, Household by TAZ – Woodford County

14. Estimated Speed

15. Congestion Level (V/C ratio) Estimation