VISSIM Lab - Solving a Freeway Bottleneck CE 454: Urban Transportation Systems, Fall 2008

Portland State University

For this lab you will use VISSIM - a microscopic, time-step and behavior based simulation modeling software - to analyze a bottleneck at the Interstate 5 and Interstate 205 southbound (SB). You should be aware that the purpose of this lab is not to teach you to be an expert in VISSIM but give you exposure to the techniques of coding, calibrating, and analyzing output from a simulation program. This exercise will necessarily be a shortened version of what you would have to do if you were going to use the tool on an engineering project. It is probably more important that you understand the building blocks of simulation presented in class rather than specific software. In practice, you will be expected to understand what the software is actually doing not just what the “black box” says. Learning Objectives

Be able to describe the basics of building a simulation model. Be able to describe why it is important to calibrate models. Analyze output generated by the software in an appropriate manner

1 Exercise Description We will look at the freeway-to-freeway directional merge of I-205 southbound onto I-5 southbound. The project boundary will be from the Tualatin Sherwood to N. Wilsonville interchanges. Note that we are only looking at southbound direction. The traffic volume at the SB merge of I-205 onto the I-5 mainline is sufficient most (but not all) days to create a bottleneck that persists both on the I-205-to-I-5 ramp and on I-5 itself. In your simulation (after the warm up period) you should see queues forming in the animations. Because of the high quality aerial photographs it is helpful to use Google Maps (maps.google.com) to view the study area. The geometry (primarily the number of lanes and locations of merge points) is of particular interest. We suggest that you make notes on the 11x17 project map about the geometry of the southbound project limits. There are two locations to note since they are little unusual and key to getting the model correct and are shown in Figure 1. The following is provided: a scaled aerial photograph for creating the network, traffic volumes during the peak period, and distribution of traffic for routing purposes, and a powerpoint handout. These data are contained in this lab document and on the class web site. Using VISSIM you will:

Create a model of the existing conditions; Calibrate the network; Obtain a measures of existing performance; Make a modification to the network that might (or should) improve the interchange such as, but not

limited to: o adding an auxiliary lane between I-205 and the North Wilsonville exit or o adding a longer acceleration lane

Compare selected measures of effectiveness for the two outcomes.

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2 lanes can exit I‐5

The ramp goes from 2 lanes to 1 before the merge with I‐5SB 

Figure 1: Merges along I-205 SB ramp to I-5

2 Getting Started Below are some tips you should know before starting the lab:

• Draw links and connectors in the direction of travel • Name each link that you create in a logical manner (see labels of links shown in the following screen

capture) • Lanes are numbered with 1 being the far right lane • Save your work periodically

2.1 Set Up Project Files Go to your N:drive and create a directory called “VISSIMLab”. All files that VISSIM creates from the analysis will be saved in these working directories. You should create 2 subfolders one called “Base” and one called “Modified”. From the class web calendar page, download the zip file, unzip in the main directory.

2.2 VISSIM Interface Open VISSIM 5.0 by going to Start > Programs > PTV_Vision > VISSIM 5.0 > VISSIM 5.0. The VISSIM interface is generally straightforward after you figure out a few quirks of inputting data. As shown in the following screen capture, all of network editing tools are on the left menu. Each icon refers to various

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network editing capabilities (links, routes, traffic volumes, speed reduction areas, etc). Mousing over them tells you what they do. The menus across the top control the simulation inputs and settings. The help is “?”.

These menu items control the simulation settings and file management 

These icons let you zoom and scale the network.  

These are the network construction icons. These can be toggled at any time. This is pointing to the network link connector icon

Help 

This area tells you whatVISSIM is expecting from you next from the mouse or keyboard. 

2.3 Change Units to English • Change Units from Metric to English • Go to View > Options > Language and Units • Change all of the units to English and click OK

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2.4 Import Background Image A quick way to construct a network is draw over the top of an aerial photograph. To do this

• Bring in the background map by selecting from the main menu: View > Background > Edit

• Click Load… and select background file (i5205.bmp) from location you downloaded from the class

web.

2.5 Setting the Scale The background map has to be scaled to get accurate results. Ideally you would know the actual scale of the drawing but for our purpose we will just scale off of two points where the distance is known. While this isn’t very precise, it is good enough for this exercise. In this example we have used Google maps1 to find the distance from the overhead sign gantry south of the N. Wilsonville interchange overpass to the overpass as shown in the following screen capture.. The distance is 1497.55 feet.                                                             1 If you want to get your own measurement, visit Google Maps: maps.google.com and click on the My Maps link, then choose the Distance Measurement Tool  

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To add the scale to VISSIM, select View, Background, Edit then Click on Scale. Using the ruler do the following:

• You will see a ruler. Find the same two points as in Google Maps. • Find starting point, then click and hold left mouse button • Drag ruler to the second point • Release left mouse button • Enter distance in feet, click OK.

It’s a good idea to save your file now. File > Save As > in the “Base” folder.

3 Creating the Network Creating the network involves adding links and connectors to build the roadway being modeled. The network will also identify the routing options for drivers. Later, you will modify driver behavior such as lane changes, headway, acceleration, and deceleration.

Always draw Links and Connectors in the direction of travel.

3.1 Creating Links

Click in the toolbar on the Links and Connectors icon . Start from the north and work your way down the network (in the direction of travel). Drawing a link is pretty simple once you get the hang of it but it involves left and right mouse clicks. Be sure to watch the status bar for a clue on what VISSIM is expecting next. We will show you first how to draw straight links (then we show you how to change alignment in the next step). Let’s draw the first link of I-5 from the top to just before the Tualatin Sherwood on-ramp. Note we are going to ignore the off-ramp at this location since it does don’t affect our bottleneck area. To draw a link:

• Place cursor over starting point for the link (shown as 1 on the 11x17) • Click right mouse button and hold

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• Drag cursor to approximately the point where I-5 and the Tualatin Sherwood on-ramp intersect (shown as 2 on the 11x17)

• Release right mouse button, after which this window will appear:

• You should name the link in a systematic manner. • For the options, Select Behavior type

o 3. Freeway (free lane selection). o The number of lanes to 3 (remember we’re only looking at the SB direction) o Lane width = 12 ft o All other defaults are good.

• Click OK and the link information is saved.

3.2 Changing the alignment of a link Not all links will be straight lines. Fortunately it is easy to adjust the links so that they align with the geometry on the aerial photograph. To add curvature to a link, do the following:

• Double click on link to be edited • Single right click anywhere on the link where you want to shift the alignment. A vertex will appear.

Now left click on the node and drag the new node to create a curve.

• Repeat by right clicking again to add more vertices as necessary.

3.3 Adding the first on-ramp At the Tualatin-Sherwood on-ramp, create a link such that it almost connects to I-5 but leave a small gap as shown in the following figure. Make sure that the two links do not meet, but come close. Before we connect the two links, we will first construct the next downstream link. Then join the 3 links with 2 connectors.

Links are connected to one another by connectors

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3.4 Adding the next downstream link Create the next I-5 mainline link south (below) the on-ramp and I5 link already created. Hopefully you noticed in your inspection of the geometry that this section has 4 lanes. For the other options, select all of the same options as in 3.1.

3.5 Connectors

This is a very important step in building a network in VISSIM. Links need to be connected to other links. It is not sufficient to place one link on top of another link in order for vehicles to continue on the other link. Instead, a connector needs to be created to connect the two links. We will be able to assign properties to these connectors. For computational reasons, it is best to keep connectors as short as possible. These connectors guide the paths of vehicles and other simulation properties.

• To create a connector, click on the Links and Connectors icon . You will create two connectors, one for the on-ramp to the downstream link and one for the mainline to the downstream link.

• Start by clicking on the ramp • In the same way you made a link, draw a connector from the ramp to the mainline in the direction of

travel. • A dialog box will appear. Lanes are numbered with 1 being the right-most lane. To make the

connector work, you need to indicate the how the vehicles from the ramp enter in to the downstream link. Clearly, lane 1 connects with lane 1.

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• Repeat for the mainline connector expect that you connect lanes 1,2,3 on the upstream link to lanes 2,3,4 on the downstream link.

• For this connector all defaults are adequate. We will come back to changing a few other values later.

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3.6 Finish Coding the Network

It’s a good idea to save your file now. Repeat this process to create the network. When you are finished it should look like the screen capture on the following page.

• After the network is complete it is sometimes easier to remove the image background for editing/debugging.

• Another helpful tip is to switch to center line view (CTRL-A) or from the view menu so that you can see links and connectors clearly.

• You can also turn on labels in the View > Network Elements menu

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4 Vehicle Inputs (Volumes) We are only going to simulate 1 peak hour (3600 seconds). In VISSIM, you only need to add a vehicle input point at the first link where vehicles can enter the network. There are only 3 links on the network where we need to add vehicle inputs (they are labeled A,B,C on the 11x17). These should be added after the network has been completely finished. These volumes are in vehicles per hour.

Link Vehicles per Hour

Interstate 5 5778Tualatin-Sherwood on-ramp 961Interstate 205 SB to I-5 SB on-ramp 2085

• To add vehicle inputs, click . • Double click on link where you want to add the volumes. • The following dialog box appears. Enter the number vehicles to arrive in vehicles per hour across all

lanes. Enter volume from chart and click OK. Note the time dialog. You could specify vehicle flows for any time window of the simulation

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It’s a good idea to save your file now.

5 Creating Routes Routes determine the paths of vehicles in the network. The routes should be set to mimic real-world decision making as close as possible. This means we want to place the “decision point” where we would expect vehicles to begin changing lanes in the real-world. For our simple network, we will have to create 2 routes. We need to tell vehicles entering the network on I5 that some will continue on I-5 SB and some will continue elect to take I-205NB. Likewise, some vehicles entering at the Tualatin-Sherwood interchange will do the same. These routes are shown on the 11x17. We will assume the following distribution of flows.

• I5SB toI5SB – 70% • I5SB to I205NB – 30% • Nyberg On-ramp to I-5SB – 95% • Nyberg On-ramp to I-205NB – 5%

To create a Route, click the route icon . • At the link you wish to be the “from” point, left click on link. The link will now be selected. • Right click on the link to on a point where you want to place the decision point. Vehicles passing this

point “Decide” which route they are going to take and begin lane changing maneuver when we tell them they need to start thinking about lane changes. For the I-5-I205 route, this decision point should be placed about 4,000 to 5000 feet away from the diverge area. Place this point at approximately the point labeled “F” on the 11x17.

• Left click on the destination link • Right click on the destination bar to get a green bar (this is the “to” point of the route). The travel

paths will be between these two points. • If the route has two options from the same starting points (and ours do!) you can add another link by

left clicking on the other destination link, then right click on the other destination link to get another green bar.

Now we need to add the information about the vehicle flows to the link. To do this:

• Left double click on the first red bar to view the Routes screen.

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This allows you to enter car destinations as a percent or as volumes. Either way VISSIM converts to relative flows so just enter the percentage volumes. The screenshot shows 70 percent of vehicles remaining on the mainlines and 30 percent exiting to the I-205 ramp.

It’s a good idea to save your file now.

5.1 Merging Sections In a situation where the two lanes become one, VISSIM will treat this as a merge and will require routing. Without routing, the vehicles would simply exit the simulation. The merge and routing set-up must be used any time there is a lane drop or a merge, such as an acceleration lane. Acceleration lane construction is important as some of you will choose to make improvements to the bottleneck area by extending the acceleration lane. This example will use the I-205 SB ramp that merges to one lane and then again merges with I-5. Lane Drop

Construct two links as shown. One link will have 2 lanes, the other will have 1 lane. Use a connector to connect the links. Remember to draw in the direction of travel.

Lane 2 will connect to lane 1. Merge

Running the simulation shows that vehicles in lane 1 of link 1 disappear. Therefore it is necessary to create a route. The route tells the vehicles in lane 1 to merge to lane 2. Route This time, there is only one option for vehicles to travel and therefore they merge.

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This shows the construction of an acceleration lane. The empty spaces between links are where you must add connectors.

The next figure shows the number of lanes for each link. Notice that the link with the acceleration lane has 4 lanes.

All of the connectors are shown. You must create some additional routes. The routes will be similar to the one created for the I-205 SB ramp lane drop. These routes will only have one choice for drivers. This is what creates “merging” in VISSIM.

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The route on the left shows the route for vehicles merging onto SB I-5. Notice that the destination link is not the link with 4 lanes. The route on the right shows the route option for I-5 SB. The purpose of this route is to keep cars from entering the acceleration lane and pass on the right.  

 

 

 

 

 

 

 

 

6 Simulation Parameters We are almost ready to run the simulation for the first time to begin calibration, debugging, and validation. IN the main menu, go to the Simulation > Parameters. You could easily set the simulation to run longer, but in the interest of time, we will run our simulation for 1500 seconds. Set the simulation speed to maximum and resolution (how often the simulation calculates each vehicles position) to 5 time steps per second. Select OK.

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Click the play button. to start the simulation. You can stop the simulation at any time without harming anything. You can use the single step play button for debugging.

7 Error Checking and Calibration While the animation is running, you should navigate around your network and observe vehicle interactions at the merge and diverge areas.

1. Follow a vehicle through the network, does it appear reasonable? 2. Does it appear that vehicles are following the routes you coded? 3. Does it appear that driver behavior at diverging point at I5SB/I205NB diverge and the I205SB lane

drop is being accurately modeled? If you notice any network errors, stop the simulation, fix the errors then rerun the simulation. You will now fix the driver behavior at the two merge areas.

7.1 Adjusting Driving Behavior Hopefully you noticed vehicles stopping on I5 trying to change lanes to get on I205. Of course, this doesn’t really happen. The same type of behavior appears to be happening at the lane drop section on I205 on ramp. The default values need to be modified to produce reasonable driver behavior. Two things are happening at the I5-I205 diverge area. First, we did not code in the appropriate distance where drivers should begin reacting to the diverge. We need to change the Lane change distance parameter for the connector which defines the distance at which vehicles will begin to attempt to change lanes (e.g. distance of signpost prior to a junction). We also need to make drivers more “aggressive” in this weaving section and in the lane drop area. In reality, drivers will be more assertive in making lane changes at diverge areas than under normal freeway conditions. We will first create the behavior type, change the link types, then modify the lane change parameter.

7.1.1 Create Weaving Behavior and Link Type  In the main menu, select Base Data >Driving Behavior. In the white “Name” section, right click and select New. Name the behavior type “Weaving” and change the car following parameters for car following and lane change as shown in the dialog boxes. Repeat this for a behavior “Lane Drop” as shown in the last two dialog boxes.

7.1.2 Create Link Type In the main menu, select Base Data >Link Behavior Types. In the white “Name” section, right click and select New. Name the behavior type “Weaving” and change driving behavior to the Weaving one you just created. Repeat this for a behavior “Lane Drop”.

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7.1.3 Change the Links in the Network to the New Behavior Types Use the link connector tool to select the two link where we want to change driving types. Double click, then change the dialog box for driving behavior as shown in the following screen capture. Repeat for the link with the lane drop.

 

7.1.4 Modify the “Lane Change” Parameter at the Connectors At the I-5-205 diverge, modify both connectors to have a lane change distance of 5000 ft as shown in the screen capture. This reflects that we want vehicles to start lane changing behavior for the diverge about 5000 ft in advance of the diverge point. The parameter can be modified to reflect real world conditions. Do the same modification for the lane drop connector, but use a lane change distance of 3000 ft.

 

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7.1.5 Check your modifications in the animation. 

Click the play button. to start the simulation. You can stop the simulation at any time without harming anything. You can use the single step play button for debugging.

8 Extracting Output for Existing Conditions The strength of any simulation software is the variety of performance metrics you can extract. Essentially, any item that you would want to know (down to each individual vehicle) could be extracted. That’s too much for us right now so we are going to keep it simple. After we setup the evaluation, VISSIM will save files with the data we have requested when the simulation has finished running. You can experiment with other data collection types but for the lab all that is required is the simple

VISSIM only writes the data collection files at the completion of a simulation run. For each data collection element, the output file is a text file with different extension. The extensions refer to the German word for the data collection element so they are not intuitive. You can see the help for the various extension names.

8.1 Network Performance We are going to collect some basic network performance data. This collects data for every vehicle in the simulation network. To do this, go to Evaluation > Files then check the “Network Performance” dialog. Use the configuration tool to collect the performance metrics you want (not emissions only work for nodes). Also, you want to use the filter to exclude the “warm up” time for the simulation. You should be able to see the approximate time in the animation when it appears that the network is loaded and vehicles are interacting properly.

Now, run the simulation. When it is complete a file with extension *.npe will be in working directory. Open this text file to see performance metrics. A sample is below.

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File: \\khensu\Home02\monserec\My Documents\Teaching\Urban Trans Fall 08\Lab\BaseConditions\final vissim interstate 5.inp Comment: VISSIM Sample Date: Friday, October 17, 2008 4:19:02 PM Simulation time from 500.0 to 579.8. Parameter ; Value; Average speed [mph], All Vehicle Types ; 30.235; Total delay time [h], All Vehicle Types ; 1.327; Total travel time [h], All Vehicle Types ; 19.355;

8.2 Getting Emissions If we want to get you will need to set up a node first, then configure the node data collection element. This is not required but for your information

9 Multiple Simulation Runs As discussed in class, starting a stochastic simulation run with the same random number seed with produce the exact same output. The “answer” of one simulation run is not very useful, we should make multiple simulation runs then calculate the descriptive statistics about our performance measures. VISSIM will save each output file with the random number seed. You are only limited by time but do a minimum of 5 runs. Make sure everything is how you want it before committing to multiple runs. To do this, choose Simulation > Multirun and complete the dialog as appropriate.

10 Modify Interchange Design Now, it is up to you to come up with a “fix”. Save existing conditions network, then “save as” a network file in the modified directory. Change the network to reflect a solution you think might work, run the simulation, and gather the output files. A suggestion is shown below.

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Figure 2: Shows new auxiliary lane between interchanges

11 Prepare Your Report The lab counts as a homework assignment. Each team of 2, students will submit the following in one zipped file named lastname1_lastname2.zip electronically that contains the following to the CE454 TA rolando@pdx.edu:

The created base network file (*.inp) The modified base network (*.inp)

A short lab report (2-3 pages) documenting your comparison of the existing conditions and the modified future performance. Please think of this report as documenting your findings for the Oregon DOT. You are free to collect other data from the simulation and include it in the report. Be sure to report the mean, st.dev and 95th confidence interval for all of your performance metrics. Credit Lab developed by C. Monsere, R. Melgoza with input from M. Dorado, N. Wheeler, T. Johnson. Last update 01/29/09.