# Transportation Systems Simulation - A Tutorial for Multi-Modal Simulation Using VISSIM/Intersection: Traffic Signal Layout

Traffic signals are traffic control devices that assign right-of-way to vehicles at an intersection with the use of green, yellow and red indications. This tutorial will walk you through how to simulate a traffic signal in VISSIM. In this tutorial the intersection of Kellogg and S. Campus near Cal Poly Pomona will be used to demonstrate the steps in creating the simulation of a traffic signal. This tutorial assumes the reader has a basic understanding of how to interact with VISSIM so it will not cover basic operations and procedures.

## Layout Design

Layout the links for your intersection using the Links from the Network Objects list. When doing this there are some important things to consider to improve your simulation:
* Layout the through lane links completely through the intersection. Do not end them at the stop bar. In fact, it is best to layout your through lanes end to end along the entire corridor you plan to simulate if possible
* For turning links, end them into the intersection past the stop bars and crosswalks. VISSIM needs to have room to place the signal heads and they do not work well on connectors
* Create a separate link for each type on turning movement. For example left turn and right turn lanes will require their own links with connectors usually placed near the upstream transitions
* To curve a link or connector, you can add intermediate points to it either by right-clicking it and selecting Add Points or by holding down Ctrl and right clicking on the link or connector
Tip: You can duplicate links and reverse their direction allowing you to quickly make both directions of a roadway. Both of these functions are available by right clicking on a selected link.
Notice in the figure below the placement of links (blue) and connectors (magenta) at this intersection. There are a few key elements to notice in the figure:
* Use about 7-10 points for left-turn connectors to get smoother curves
* Use about 5-7 points on right-turn connectors for smoother curves
* Use about 3-5 points for transitions from through lanes to turning pockets
* Make sure you have a connector connecting every possible lane change or turning movement condition
Tip: You can toggle the centerline view to lane view using the centerline button just above the main layout window.

## Conflict Areas

Conflict areas are regions where you have overlapping links and connectors. Not every conflict area requires a priority. For example, at a signalized intersection, the traffic controller should control when vehicles pass through the intersection thus eliminating the requirement for prioritizing most conflict points. Here are the key points for conflict areas:
* Prioritize all conflict areas where vehicles may be allowed to travel on both links at the same time. For example at right turns.
* Conflict areas are color coded:
** Yellow = No determination has been set
** Green = First priority
** Red = must yield to existing traffic
* Priorities can be assigned to either link or connector, left as permissive or set to undetermined.
* Give the priority to the direction that would have it in real life. As a rule, right turns typically yield to through movements, thus the through movement should be given the priority (even with RTOR movements).
Tip: There will be several conflict areas that overlap. Make sure the highlighted conflict area is the actual conflict area you need to adjust.
* Float your mouse over each conflict area and it will highlight. Right-click on the area to bring up the shortcut menu as shown in the figure below.
* Select the appropriate “Set Status to # waits for #”.
* If you make a mistake, simply right click on the conflict area again and change it.
When all of your right turn conflict areas have been prioritized, it should look similar to the figure below.
Back to Main Page