[Step 4] Adding a Priority Control for Permissive Movements
Even though most conflicts at the signalized intersection will be controlled by the signal, there are still a significant number of conflicts which are not directly controlled by the signal. It is mainly caused by permissive movements such as right-turn-on-red (RTOR), permissive left-turns, etc. In this step, two different types of objects including 1) stop sign and 2) conflict area will be used. Details on each object can be found in the following section.
When a vehicle is allowed to make a right turn during a red, the driver is supposed to treat the signal as they would a stop sign; therefore, they will come to a complete stop, then check for conflicting traffic. In Vissim, an approach is configured for RTOR using a stop sign object. Stop signs can be configured to work in conjunction with a specific signal group.
Note that stop signs must be used along with conflict areas so that the actual conflict that happens after the stop sign can be controlled. Follow the steps below to add stop signs for RTOR:
1) Double click on right turn signal head object to edit. It will open signal head window.
2) Since this signal head will not be obeyed by vehicles but only be used for visualization purposes, unselect “All vehicle types” and select vehicle class which does not travel on this link (i.e. Pedestrian).
4) Click “Stop Signs” in the network objects window as shown in the figure to be in stop signs insert mode.
5) Left click to select the link or connector where the stop sign will be placed. This stop sign should be overlapping to where right turn signal head is placed. If the right turn lane is shared with another movement (i.e. thru movement), you may will need to have a longer connector so that each turn can have its own signal heads placed.
6) Press <CTRL> + Right click to place stop sign and Stop Sign window will open as shown below.
7) Go to “RTOR” tab and check “Only on Red” and specify SC and SG connected to this RTOR operation. (It typically will be SG for concurrent thru movement.)
8) Click “OK” button to save.
Exercise (Download Example Files)
1) Open “Step_4-1_Add_Priority_Control_Stopsign” folder and load “Stringfellow_Rd_ Step_4-1.inpx” file.
2) Click “Stop Signs” in the network objects window to be in stop signs insert mode.
3) Zoom into signal heads for NBR or SBR and left click on the link (or connector) where the signal head is placed to select.
4) Press <CTRL> right click on top of right turn signal head to place stop sign.
5) Under “RTOR” tab, select appropriate SC-signal group to match up with SG assigned to right turn signal head.
6) Click “OK” button to save.
7) Select signal head for right turn movement below stop sign placed above and double click on it to open “Signal Head” window.
8) In “Vehicle classes” section, uncheck “All vehicle types” and check “50: Pedestrian”. This way, this signal head will not be obeyed by any vehicles, but you can still observe signal status changes while the simulation is running.
9) Repeat same steps for those places where RTOR is allowed.
10) Click “Save” icon and “Run” simulation and observe.
In order to manage conflicts caused by permissive movements such as permissive left turns and RTORs, it is necessary to add objects which can manage conflicts in between unsignalized (permissive) movements. In traffic there are generally three types of conflicts; merging, diverging and crossing; however, merging will be the most common type of conflict that will be caused by permissive movements at the signalized intersections.
Conflict area objects in Vissim should be used to handle these unsignalized conflict management cases as shown in the figure on the right. On the contrary, it is also important to not place conflict areas for those movement conflicts which are managed by the signal (i.e. protected left turn and opposing thru movements).
Conflict areas are automatically generated any time two or more links occupy the same space. A conflict area has three states:
- Passive (displayed yellow)
- Active (movements are displayed red and green depending on priority setting)
- Undetermined (both links are displayed red and it is first-come-first-served)
As vehicles on the minor street (displayed as red) approach the conflict area they determine whether there is a large enough gap for them to pass through the conflict without disturbing the major movement traffic. If need be, a vehicle will change speed or come to a complete stop to avoid a “crash”. Vehicles on the minor street will attempt to minimize their delay so if they do not need to stop. Conflict areas use a few parameters to calculate acceptable gap including “front gap”, “rear gap”, and “safety distance factor” however these will not be discussed in this guidebook. Refer to Vissim user manual for details on each parameter. All overlapping sections of links will create a conflict area automatically and simply require the user to determine priority scheme. Follow the steps below to add conflict areas:
2) Zoom into the conflict area to define priority scheme.
3) Left click on the conflict area you would like to update priority scheme on. If you cannot select specific conflict area that you want, left click as close as possible and press “Tab” key to select underlying object(s).
4) Press <CTRL> + Right click to change priority scheme and repeat until you see desired priority scheme.
5) Repeat for all permissive movement conflicts for each intersection, including all vehicle-to-pedestrian conflicts as shown below.
Exercise (Download Example Files)
2) Click “Conflict Areas” in the network objects window to be in conflict area insert mode.
3) Zoom into south west quadrant of the intersection as shown in the figure.
4) Left click on any conflict (yellow area) to update conflict control status to a realistic combination. If it cannot be selected in the network editor window, click as close as possible and use the “Tab” key to toggle.
5) Update conflict configurations for EBR movement and pedestrian crossing and SBT by pressing <CTRL> + right click.
6) Complete all priority settings for permissive movements and click “Save”.
7) “Run” simulation and observe.