Visual-tm: Environmental Models
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The environmental model
The Environmental Model in Visual-tm produces an output giving a number of environmental characteristics for each link in the network. It is called up using Environmental Models on the Models Menu, which displays the following:
Click View Coefficients, which expands the form, and then Read Coefficients to display the following:
The coefficients take a while to load, after which they may be viewed on the other tabs. The coefficient file is called PollutionCoeffsNew.csv and is located in the Visual-tm Templates directory. This is the file that will be loaded if no file name is entered in the Input Coefficients File text box. The parameters on the tab shown above are for the user to set and are covered in the explanation of the calculations.
The Pollution Coefficients tab has the coefficients that are used to calculate the amount of each of five pollutants that are produced on each link.
For each pollutant there are ten sets of coefficients. There is a separate set for each of five vehicle types with two sets for each vehicle. One is for petrol vehicles of that type and the other is for diesel. Some rows are empty e.g. petrol HGVs as the vehicle type does not exist.
The Road Categories tab lists the Accident Type and PIA value for each of 41 link types. The use of these is described later.
There are fifteen Accident Types, which are each listed on the Accident Types tab along with factors related to their frequency.
The Stop Start coefficients are used to calculate the percentage of PCU’s starting and stopping on the link. There is a set for each of nine speed bands with the speeds for the band given on the record.
To run the environmental model an input file is needed, which is an output assignment file from Visual-tm. This has traffic flows on each link and is therefore needed to calculate the environmental consequences of the traffic. Select the required file and enter a name for the output file. Click the Process File button.
The Ashtest test data project includes an input assignment file and output environmental file. These are called pbAss.dat and Environmental.csv and are in the Project’s \System\Working directory. The output file has 21 fields with the first ten being input data to the environmental model, starting with the A Node and B Node for the link and the remaining eleven are the output fields.
The first five output fields give the amount of each of the five pollutants produced on the link. The values are in grams per day, assuming that the input assignment file had daily flows. The next three columns are for the numbers of accidents on the link, divided into Fatal, Serious and Slight and the values are annual. The reason why these fields are all zero in this example is that the Ashtest data does not contain the Link Type data that is necessary for the accident calculations. The Link Type number has to be entered in the Accident Category column on the Links sheet in the Network data file and this field is not present in the Ashtest data, although it is in the Template files. Another required field for accident calculations is the Urban or Rural field. This should have a 1 in it if the link is urban and a 0 if it is rural.
After the Accident fields is the Fuel Consumption, which is given in litres. This is the daily consumption on the link, assuming that the input flows are daily. The next field is the number of stops, which gives the number of vehicles that start and stop due to the effects of congestion. The final field is the Link Performance Index, which is explained in chapter 29 of the Visual-tm manual.
Editing the coefficients
The coefficients on the grid may be edited and the new ones saved back to a file. To do this, click on the cell that you wish to edit and its contents will appear in the Edit Cell text box. Change the number to the new value and the changes will appear in the cell on the grid. When you are ready to save the file, enter the name of the file to save to in the Output Coefficient file text box and click the Save File button. Note that it will not let you overwrite the PollutionCoeffsNew.csv file in the Templates directory.
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