Travel Demand Forecasting: Traffic Assignment CE331 Transportation Engineering

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

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

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

All or Nothing Assignment O D 10 min 12 min 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) veh 0 veh

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

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

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

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).

Example 1 O D t 1 = 6 + 4x 1 t 2 = 4 + x x 1 (thousand vehs) x 2 (thousand vehs) At equilibrium, let t 1 = t 2  6 + 4x 1 = 4 + x 2 2 x 1 + x 2 = 4.5 x 1 = 1.6 x 2 = 2.9

Population, Household by TAZ – Woodford County

Estimated Speed

Congestion Level (V/C ratio) Estimation