Defining ramp metering controllers

You can define a ramp metering controller that controls the vehicle input from one link to another. For example, you can use it to model an access ramp that is input-controlled during increased traffic and leads to a section of a highway or expressway that is prone to traffic jams.

Ramp metering controller method ALINEA

The basis of the ramp metering controller is a signal controller type that implements the ALINEA ramp metering controller method and provides the necessary parameters. So there is no need to program the required run control yourself. ALINEA-compliant, detectors detect the vehicles on the section downstream of the access ramp that are prone to congestion as well as the vehicles at the signal head on the access ramp. They then control the signal head accordingly.

Optionally detecting and dispersing queues

Optionally, you can place a detector at a desired distance upstream of the signal head. This detector determines on the basis of the occupancy rate whether vehicles are backed up at the signal head and can trigger the queue dispersion through the signal head. Since this controlled queue dispersion is optional, the attributes for configuring the queue dispersion may contain empty values. Due to empty values, no queue resolution takes place in the following cases:

• If the attribute DetPortNoQ (Queue detector port number) of the signal controller does not contain a value.
• If the attribute DetPortNoQ contains a value, but one of the attributes does not contain a value:
• OccupQDispAct (Queue dispersion activation occupancy)
• OccupQDispDeact (Queue dispersion deactivation occupancy)
• MetRateQDisp (Meter rate queue dispersion)

Consideration of detectors depends on attributes

The number of detectors assigned to the signal controller can be found in the attributes DetPortNoQ (Queue detector port number) and DetPortNoRamp (Ramp detector port number) of the signal controller.

If you delete this number, Vissim will take into account the detector as if it were on the highway or expressway. Thus, deleting the number of a detector in the mentioned attributes does not deactivate this detector.

Notes on calibration

Measure the occupancy rate in the relevant section of the highway or expressway.

• Define a data collection point at the beginning and end of a detector placed on the highway.
• Combine both data collection points into one data collection measurement.
• The Occupancy rate attribute of the data collection results then shows the actual occupancy rate.
• If vehicles are shorter than the detectors, place more data collection points on the detector accordingly. Combine them into a data collection measurement.
• The occupancy rate measured should be approximately equal to the Desired occupancy of the ramp metering controller, unless the queue dispersion is effective and the Desired occupancy is somewhere between the highway traffic without access traffic and the highway traffic + access traffic without the ramp metering controller (unregulated access). If the Desired occupancy is outside this range, observe the corresponding limit.

Evaluation of the detectors

The following applies to the evaluation of the detectors on the highway or expressway: The occupancy rate in a time interval is the arithmetic mean of the occupancy rates of the individual lanes.

Creating a ramp metering controller

During the definition of the objects in the network editor, the Short help is displayed, in which the necessary steps are described. You can show and hide the short help (Short help button in the Network editor toolbar).

1.  Make sure that:

• The links for the section prone to congestion are defined.
• The links and connectors for the access ramp are defined.

2.  When selecting the section in the network editor, make sure these links are not displayed in the upper area, as that is where the attributes and necessary steps are displayed during the definition stage.

3.  On the network editor toolbar, on the very left, click the Create ramp metering facility button .

• The gray window Generate ramp metering controller opens on the left side of the network editor and displays the predefined attributes of the ramp metering controller.

• The white message box Generate ramp metering controller opens on the right and shows the first steps required for definition.

• The mouse pointer changes to a cross-hair.

In the next step you define the detector and the signal head on the access ramp. Ensure that the position of these two network objects and the acceleration lane of the access ramp, downstream, are long enough to allow vehicles approaching the signal head to accelerate to the average speed applicable on the highway or expressway.

4.  Click the desired position on the link of the access ramp.

Vissim inserts a detector and a signal head.

In the next step, you define the detector on the link prone to congestion, which measures the volume there. When determining the position, make sure that it is downstream of the access ramp:

5.  On the link of the highway or expressway, click the desired position.

Optionally, you may define a detector for the access ramp, which measures the queue upstream of the signal head of the ramp metering controller.

6.  In the gray window Generate ramp metering controller, select GenQDet and click the desired position on the access ramp.

7.  If desired, in the gray window Generate ramp metering controller, edit the following attributes:

Element	Description
MetEvalInt	Metering evaluation interval[s], default value 60
MetRateQDisp	Meter rate queue dispersion: Target flow rate with activated queue dispersion [Veh/h] per lane, default value 900
OccupRate	Occupancy rate target value: Target value of the occupancy rate in normal operation in %, default value 25
OccupQDispAct	Queue dispersion activation occupancy: Occupancy in %, at which the queue dispersion is activated, default value 50
OccupQDispDeact	Queue dispersion deactivation occupancy: Occupancy in % at which the queue dispersion is deactivated, default value 10
RegPara	Regulatory parameter: Regulatory parameter of the ALINEA method [Veh/h] per lane, default value 70
GenQDet	Generate queue detector: Optionally generate a queue detector which measures the queue upstream of the signal head of the ramp metering controller
QDetPosBefStop	Queue detector - position before stop: Distance between the start of the queue detector and the signal head, default value 100

8.  In the gray window Generate metering controller, click  to confirm your choice.

You can edit the attributes of the generated network objects in the attribute list of these network object types (Attributes of signal controllers), (Attributes of detectors), (Attributes of signal heads).