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Proposed Model
Considering the above objective, a new model is proposed consisting of four stages: generation of base year O-D person trips matrices, base year travel demand modeling, forecasting of O-D person trips matrices, and the rail transit corridor identification (Fig. 1). These stages are discussed below.


Fig.1- Proposed Four Stage Rail Transit Corridor Identification Model
Base Year O-D Person Trips Matrices Generation
The first stage in the model consists of generating the base year O-D person trips matrices. To forecast the ridership, the model only requires total number of person trips (irrespective of the mode used) from each zone to every other zone, for the planning year. Hence, the base year O-D matrices are to be generated for total person trips only. The freight modes are not to be considered while creating the matrices because the model requires a desired share for public transport, which can be obtained by splitting the person trips made only by the passenger modes. Accordingly, the O-D matrices are generated for base year. In this stage, the model requires home interview survey, screen line, cordon line, network, and O-D survey at passenger terminals data. Following are the steps involved in the procedure: -

Step 1: From the home interview survey data, the expansion factors for each zone are calculated by dividing the total household by the sample households.
Step 2: The O-D matrices are extracted from the home interview travel survey data, by counting each trip from the person's origin to final destination as one person trip. This is to be done separately for HB, I-E, and E-I trips. The sample flows are then expanded to zonal level by multiplying them with the expansion factor for each zone.
Step 3: The O-D flows obtained from home interview survey data are supplemented by the O-D flows obtained from O-D cordon survey, and O-D survey at passenger terminals. This gives the final O-D matrices with all person trips (HB, NHB, I-E, E-I, and E-E).
Step 4: To validate the matrices, the combined O-D matrix of all person trips is loaded on to the base year network (after converting it to peak hour matrix using the appropriate average daily to peak hour ratio) using user equilibrium approach. While assigning the O-D flows, the capacity of links is used in terms of passengers per hour per direction (pphpd), which can be obtained by multiplying the capacity of each link in PCU per hour by the average car occupancy rate. Also, it is necessary to specify the link performance function and its parameters before the assignment can be done on the network because the user equilibrium process assigns the flows iteratively based on the link performance function. A link performance function is a mathematical description of the relationship between speed or travel time and link volume, a typical example of it is Bureau of public roads (BPR) formulation:


Where, t is congested link travel time, t_f is link free-flow travel time, v is the link volume, c is link capacity, and a, b are calibration parameters. Using the above set-up, the assignment can be done. Comparing the assigned and observed link flows across the screenlines then validates the matrices.
Step 5: Finally the validated daily O-D person trips matrices for base year will be obtained, which will be used for the base year travel demand modelling, as described below.

Base Year Travel Demand Modelling
The base year travel patterns are to be modelled as accurately as possible and the models along with the horizon year planning variables and network information can then be used to forecast the trips in horizon year. This stage involves development of trip end and trip distribution models.

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