# Transportation Systems Simulation - A Tutorial for Multi-Modal Simulation Using VISSIM/Freeway: Geometric Network Design

Designing a VISSIM network in accordance to the background map involves adding links and connectors to build the roadway. The network will also identify the routing options for vehicles.

## Creating Links

* Click on the Links toolbar under “Network Objects” (see Figure below).
* Second, Right Click a point where you want to start your network, then select Add New Link (Shortcut: CTRL + Right Click).
* A “Link Data” window will appear where you can define:
** The name of the link
** Behavior type: Freeway (free lane selection) or Urban
** The number of lanes
** Lane width = 12 feet
* Leave all other options as the default settings
* Click OK
* Click and Drag either end point to your desired final location
TIP: You can access/change the “Link Data” by Double Clicking on any link. Pay close attention to the direction of the arrows located at the edges of each link. The arrows indicate the direction of traffic flow. It is highly recommended you create all basic links first, with their approximate start and end locations. Adding lanes, reversing the direction or generating adjacent lanes in the opposite direction can be done by Right Clicking the link and selecting your desired option (see Figure below).

### Changing a Link's Alignment/Curvature

You can change the alignment/curvature of links so they can align with the geometry on the aerial photograph (see Figure below).
* Right Click anywhere on the link you want to add a vertex and select Add Point (Shortcut: CRTL + Right Click)
* You can also Right Click anywhere on the link and select Generate Spline
* A new window will appear. Enter the number of intermediate points you wish to add. Then select OK
* Click, Drag and Drop each point to your desired location(s)
TIP: You can roughly outline a Highway segments with 3 or more points, then use the “Generate Spline” function to add more points (see Figure below). This will automatically smooth out the curve. You can continuously add intermediate points or replace all existing points by Unchecking “Keep current intermediate points”.
WARNING: DO NOT add too many points to a curved highway segment or to your entire network. It will slow down the application and make it difficult to make future alignment adjustments.

## Creating Connectors

Connector guide the path of each vehicle. Links must be connected to other links for continuous travel. A connector is the tool to connect two links. For computational reasons, it is best to design connectors not too long and not too short.
* Click on the Links toolbar under “Network Objects”
* On the upstream link and precise lane, Hold CTRL+ Right Click
* Drag the cursor to the desired downstream link and lane
* Release the right mouse button, after which a “Connector” dialog box will appear (see Figure below)
* Make the right connection between two links
** Note: lanes are numbered with 1 being the right-most lane of travel (slow lane). To make the connector work, you need to indicate how the vehicles from the ramp enter/exit into the downstream link
* Click OK to finish
TIP: If a two lane highway segment merges into one lane, first draw a two lane link next to the one lane link. Then connect the links with two short connectors.

## Finalizing the Network

Repeat the above process to create the entire network. After the network is complete it might be easier to hide the image background/map for editing purposes. You can toggle the map or background off/on to view only the network (see Figure below).
* Select the Backgrounds tab, located on the bottom left corner above the "Quick View" window
* Click on the eyeball
* To hide the background map, Click on the “Globe” in the Network Editor
* Toggle off/on the “wireframe” to clearly view the map or background (Shortcut: CTRL+A)
TIP: Name every link and connector with a unique and descriptive abbreviation. This will help to easily identify the link. For example:
* The 60 Eastbound Mainline link will can be named (60EB ML)
* The Northbound Fullerton On-Ramp can be named (NB Fullerton ONR)
* The NB Fullerton ONR to 60EB ML Connector (NB Fullerton CON)
* You can also number (1,2,3…) multiple links with the same name (60EB ML1, 60EB ML2, 60EB ML3...)
The content above is also available as a video. Click the following YouTube link: VISSIM v7 Tutorial Part 3: Geometric Network Design
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