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. If transport systems are added to or removed from the link, this is also done automatically on the lanes assigned to the link. The rule is that the set of transport systems on the lane must also be included in the transport system set of the link. Likewise, it automatically ensures that the transport system set of through lanes is consistent at both nodes.
The transport system set of lanes is considered in the simulation-based assignment, as well as in the export to PTV Vissim.
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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.
As with lanes, the transport system set allowed on a lane turn is, by default, equal to the transport system set on the underlying turn or main turn. If transport systems are added or removed on the turn or main turn, this also happens automatically on the assigned lane turns. The transport system set of a lane turn must always be included in the transport system set of the turn or main turn.
The transport system set of lane turns is considered in the simulation-based assignment as well as in the export to PTV Vissim.
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)).
