In macroscopic traffic models, an at-grade junction is represented by a node (point object) with turns. Macroscopic modeling, however, does not reveal anything on the detailed geometry or the geometric design of a junction. Nearly the same applies to the node control. The optional extension of the Visum network model by node geometry and junction control can be used in the following fields:
- Calculating the performance at a node
- Considering node impedances during assignment
- Providing entire junctions for the microscopic model Vissim
A node geometry consists of the items node legs, lanes, lane turns, detectors, and crosswalks. If a signal controller is allocated to a node, its data refers to the node geometry. By default, no geometry data is provided at a node. It is generated not until the first access.
Legs
The principal elements of the geometry are the legs. A node/main node can have up to sixteen legs. The set of legs is determined by the orientations of the incoming and outgoing links (Network objects of the junction model). For each used link orientation, exactly one leg is generated. Legs can thus either consist of an incoming link and its opposite direction, or of an incoming one-way road and an outgoing one-way road.
Legs can have a center island, a channelized island, or both. For a center island to exist, the center island length and width both need to have a value > zero. For a channelized island to exist, the channelized island length needs to be > zero. The Stop line position attribute is only used for the export to Vissim. Legs also possess a set of lanes.
Lanes
There are incoming lanes and outgoing lanes, as well as through lanes and pockets. The number of through lanes at a leg cannot be changed. It is based on the set number of lanes at the links which underlie the leg. Therefore, if the incoming link of the leg has three lanes (Number of lanes attribute on the link) and at least one transport system, the leg features three incoming through lanes. If the number of lanes at this link is changed, the number of through lanes at the leg will be adjusted automatically. We recommend double-checking the adjusted geometry data after such modifications. Since at least one open link underlies each leg, each leg features at least one through lane.
The number of lanes at a leg can be changed by creating pocket lanes (pockets). Pocket lanes always refer to a through lane on which they originate (origin lane). In contrast to through lanes, pockets can be removed again. For pockets, a length can be specified. This is used during Vissim exports and for specific methods of impedance calculations at nodes.
By default, the transport system set permitted on a lane corresponds to the transport system set of the underlying link. For pockets, the transport system set of the origin lane is used by default.
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Note: The numbering of the lanes differs from the one in Vissim. |
Lane turns
A lane turn connects an incoming lane with an outgoing lane. When generating a geometry automatically, a set of lane turns is also generated automatically. In order to define a lane turn, the turn or main turn between the link underlying the incoming lane and the link underlying the outgoing lane must be open. This means that it needs to have at least one transport system.
It is usually not desired that lane turns intersect. Two lane turns, for example, intersect if one of them makes a left turn on a right lane and the other one goes straight on a left lane. This is yet possible and desired if the left turn is a PrT turn and the other one a PuT turn. In this way, a tram can, for example, be modeled in central position.
The set of lane turns basically determines the results of the node impedance calculations at a node/main node.
Crosswalks
Crosswalks are objects that connect the sides or the islands of a leg per direction. Depending on the combination of islands at a leg, you can define up to six crosswalks. If the node leg e.g. has a center island (i.e. its center island length and width are both > zero) and a channelized turn, six crosswalks can be defined: One between a side and the center island, one between the center island and the channelized island, one between the channelized island and the other side, and one each in the opposite direction.
Crosswalks are exported to Vissim. For crosswalks, a pedestrian volume can be specified. This is relevant when calculating the node impedance using ICA (Intersection Capacity Analysis according to the Highway Capacity Manual (ICA)).
Leg templates and geometry templates
In order to ease the input, leg templates can be used for legs. With the aid of leg templates, a set of predefined lanes, lane turns, and crosswalks are generated at a leg. Contrary to earlier program versions, the object's reference to the template is not kept when using leg and geometry templates. Previously, legs could not be edited if they were allocated a template. Now, templates are used exclusively to define leg and node geometries.
For the generation of leg templates, existing legs are used. The attribute values of the leg are transferred to the template. They can, however, be edited later on. A leg template consists of lane templates. If a leg template is generated from a leg, the lanes of the leg are used as a model for the lane templates. The lane templates can also be edited later on.
Leg templates can only be used at geometries of 3 or 4 legs. The data must match so that a leg template can be used at a leg. If a template is suitable for nodes with three legs, it can thus not be used for legs at nodes with four legs. The number of incoming and outgoing lanes of the leg and of the template must also be identical.
Contrary to leg templates, geometry templates can be applied to all legs of the node. They can also be used exclusively at nodes with 3 or 4 legs. A geometry template is made up of several leg templates. When using a geometry template, the leg templates are applied to the legs of the node. To determine which leg template is to be used at which leg, a reference leg must be specified for the template. Geometry templates can only be used if at least one valid reference leg exists, so that all leg templates can be used in the right order for all legs at the node.
