In this example we will see which intersections have poor levels of service and mitigate the congestion at intersection #4.
Select the Mitigated Conditions scenario. During scenario definition (Step 2.1), we defined separate data sets for Command data and Geometry data. All other data sets are common to both scenarios. In Step 3.6 we copied the Base scenario geometry to the Mitigated scenario. Therefore, at this stage the analysis results of the two scenarios are the same.
Place the cursor over one of the intersection tools on the left side of the screen (Signalized, Four-way Stop, or Unsignalized) and RightClick. Here you select whether you want to see base or future levels of service (LOS) displayed next to the node index number. For this example select Future. The levels of service are shown with a “*” sign to indicate that the conditions displayed are for the future case. (The future case includes the base traffic volumes plus the traffic added by the zones which have been selected for analysis.)
Move the cursor to the desired intersection (node #4), then RightClick to bring up the intersection pop-up menu. Select LOS Mitigate and the “LOS Mitigate” screen will appear. In the lower left corner of the screen, you can select the base or future volume alternative. We want to mitigate the future LOS, so select Future Alternative.
In the upper right corner of the screen is the calculation mode field. Select the “Calc on Keystroke” option, which will cause TRAFFIX to recalculate the LOS results after each keystroke. This is useful when you are only changing single character fields like the “Lanes” fields.
Use the Tab key to move progressively from field to field. Use the mouse cursor and click in any desired field to move to that field directly.
Notice that there is a row in the bottom half of the screen with asterisks in some of the fields. This row shows you which movements are critical. The row above that shows the volume/saturation ratio for each movement.
The overall critical volume/capacity ratio and delay are shown near the bottom of the screen. These values tell you whether or not your proposed mitigation will work or not.
The idea of mitigation is to identify the critical movements which determine the critical v/c ratio and provide additional capacity for these movements. For intersection #4 of this example, the v/c ratio is 1.110, the level of service is “F”, the intersection delay is 66.6 seconds, and the most critical movement is the WB right turn movement. Intersection #4 would be in a state of overflow. We will try alternative mitigation measures for each movement to reduce critical v/c ratio, delay and level of service.
Move to the Southbound left lanes field. Change it from one to two lanes and notice the effect on LOS. You will see no improvement. The critical v/c ratio has not changed and the average delay is still in the LOS F range. Change this field back to one lane and let’s try another improvement.
Move to the Northbound through lanes field. Change it from one to two lanes and note the effect on LOS. This time you will see some improvement. The critical v/c ratio is now 1.020 (which is still too high) and the average delay is now in the LOS D range. Change this field back to one lane and let’s try another improvement.
Move to the Westbound right turn lanes field. Change it from one to two lanes. You will now see significant improvement in the v/c ratio, level of service, and intersection average delay. With this mitigation, the critical v/c ratio would be 0.799, the level of service would be “B” with average delay of 13.5 seconds. This mitigation measure is thus more effective than either of those tested in steps f and g.
Before you exit this screen, you can enter a description and cost for the mitigation measure for use in the impact fee analysis. To save the changes to memory and exit, click OK. After you have exited the LOS Mitigate screen remember to save the database (Step 2.5), so that the changes will be saved to disk. Note: this will overwrite the original SAMPLE.TFX database with the changes you have just made. If you want to keep the original database in its distribution state, use the File…Save Database As option to save your changes in a new database with a different name.
Select the Tools…Traffix for DOS (Forecast) option and run the forecast module for the Mitigated scenario. When the forecast is done, view the contents of the Report file.