TRANSCAD教程5:DEMAND FORECASTING: TRIP DISTRIBUTION
2012-10-23 10:39:48 来源:tranbbs 作者:佚名 评论:0 点击:
Task 4. DEMAND FORECASTING: TRIP DISTRIBUTION
4.1 Base Gravity Model Application
The output of the Task 3 Trip Generation procedure serves as one of two basic inputs to the Task 4 Trip Distribution procedure. The conventional Gravity Model distributes trips from production zones to attraction zones as a function of zonal productions and zonal attractions, both outputs of trip generation, and of some function of travel impedance. This measure of spatial separation is typically taken as uncongested automobile network travel times, although most model applications feed back congested travel times from traffic assignment (and often reflect alternate modal travel times). The gravity model can be singly-constrained (followed by attraction balancing), or doubly constrained, where all productions and attractions remain balanced in the estimated trip matrix. TransCAD can either calibrate a Gravity Model (i.e., estimate the function of travel impedance, or "friction factors") or simply validate or forecast with an existing calibrated model. A set of estimated friction factors for Miasma Beach will be used for this exercise.
4.1.1 Open Input data
To execute trip distribution analysis in TransCAD, the Travel Impedance Matrix (skim tree matrix) that was created in Task 2 must be openned. Go to File / Open and select the drive and directory where the Task 2 results were saved. Select Matrix as the file type; a list of matrix files created in Task 2 should appear. Select skimtree.mtx and click OK. Also open the TAZ dataview by first opening the TAZ Geographic file and then clicking on the New Dataview button. This table contains the production and attraction values that you estimated for each TAZ in Task 3.
4.1.2 Appending Friction Factors
The trip distribution model requires not only production and attraction data, but also calibrated friction factors for the gravity model. Table 5 provides friction factors by trip purpose which were calibrated in an earlier modeling effort in Miasma Beach. Travel times which are greater than the table maximum utilize the F-factor for the last category. To create this friction factor matrix, first enter the friction factor look up table (Table 5) into TransCAD, and then create a new matrix file (to be filled with friction factors). TransCAD will read the travel impedance matrix (skimtree.mtx) and look up Table 5 to fill the new matrix with friction factors.
Note: This model application will not utilize "socio-economic adjustment" K-factors.
Table 5. Existing Miasma Beach Friction Factors (2004) --------------------------------------------------------- Friction Factor (Fij) Time (min.) ------------------------------- HBW HBO NHB --------------------------------------------------------- 1 25 50 30 2 18 35 25 3 15 25 20 4 13 20 17 5 11 18 15 6 10 16 13 7 9 14 11 8 8 12 10 9 7 10 9 10 6 9 8 11 5 8 7 12 4 7 6 13 3 6 5 14 2 5 4 15 1 4 3 16 1 3 2 17 1 2 1 18 1 1 1 19 1 1 1 20 1 1 1 ---------------------------------------------------------
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4.1.3 The Friction Factor Lookup Table
Step I: Create the Friction Factor Lookup Table
Go to File / New. In the Create New File dialog box, choose Table as the File Type. Under "Choose a Type of Table", select dBASE / FoxPro. Click the [Add Field] button and enter the following under Field Information:
Name: Min_TT
Type: Integer
Width: 4
Check the Index box.
Add three additional 4-digit integer fields with names (uncheck the Index box): HBW_FF, HBO_FF, and NHB_FF. Click OK and save the file as fflkup.dbf (this file is a dBASE file). An empty table with 4 column headings has been created. This table must be filled with the friction factors from Table 5. Go to Edit / Add Record. An empty row appears: enter the first row of Table 5. Continue to add the remaining records.
Step II: Create a New Matrix File
A new matrix file must be created to hold the appropriate friction factors for the gravity model application. Make TAZ the working layer. Go to File / New and choose Matrix File as the file type. In the Create New Matrix File dialog box, enter the following:
Name: Friction Factor Shell
IDs are in: ID
Rows from: All features
Columns from: All features
# of matrices: 3
Click on Matrix 1 to highlight it then change its name (in the text box) to HBWFF. Change the names of Matrix 2 and Matrix 3 to HBOFF and NHBFF, respectively. Click OK and save the matrix file as ff.mtx. Note that a matrix file can contain more than one matrix (for example, ff.mtx contains three matrices: HBWFF, HBOFF, and NHBFF).
Step III: Fill the New Matrix with Friction Factors
The newly created matrix must be filled with friction factors from the look-up table. For each of three trip purposes, the friction factors for a particular pair of origin and destination zones are determined by finding a Min_TT in the look-up table fflkup.dbf that matches the travel impedance between this pair of zones (based on the skim tree matrix). Travel impedances which are greater than the table maximum use the F-factor for the last category.
To fill the new matrix with friction factors, check that the following files are open (If closed, go to File / Open):
Friction Factor lookup table (fflkup.dbf)
Friction factor shell (ff.mtx)
Skim Tree matrix (skimtree.mtx)
Go to Planning / Trip Distribution / Synthetic Friction Factors. The Create Friction Factor Matrix dialog box will appear; enter:
Matrix File to Fill: Friction Factor Shell
With HBWFF highlighted, enter the following under Change Information:
Check "Fill this matrix" box
Under Impedance function, choose Discrete (F-Factors lookup)
View: FFLKUP
Impedance: Min_TT
F-Factors: HBW_FF
Under Impedance Matrix:
Matrix file: Skim Tree
(The skim tree matrix created in Task 2)Matrix: Matrix 1
(The default name of the skim tree matrix. A matrix file can contain more than one matrix.)Click HBOFF under matrix to highlight it, and apply the same procedure using HBO_FF as F-Factors). Repeat for NHBFF as well. Click OK when finished.
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TransCAD uses the friction factor lookup tabl
相关热词搜索: TR RA AN NS SC CA AD D教 教程 程5 5: :
上一篇:TRANSCAD教程4:DEMAND FORECASTING: TRIP GENERATION
下一篇:TRANSCAD教程6:DEMAND FORECASTING: TRAFFIC ASSIGNMENT

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