Using hybrid simulation

During dynamic assignment, you can use mesoscopic simulation to simulate your entire network. At the same time, you may also use microscopic simulation to simulate one or several areas of your Vissim network. This combination allows you to create a Vissim network of the size required for your particular use case and perform a detailed analysis for the results of the relevant network parts based on microscopic simulation.

Sections define the areas of the microscopic simulation

To define the areas you want to simulate microscopically, you define sections (Modeling sections). You select these sections in the simulation parameters for mesoscopic simulation (Selecting sections for hybrid simulation). Within the sections, vehicles are moved according to the algorithms of microscopic simulation. The settings of mesoscopic simulation do not apply for these sections. For vehicles traversing between the two areas, the mesoscopic simulation settings continue to apply. Accordingly, properties such as the desired speed are kept. Vehicles in the mesoscopic and microscopic areas of the hybrid simulation take toll pricing into account.

Please note the information on how to define sections (Modeling sections). As the calculation basis for vehicle movement changes significantly at these transition areas, unrealistic vehicle behavior may occur, particularly if the edges are very short.

Transition between areas of microscopic and mesoscopic simulation

The following figure shows a schematic drawing of the transition area from micro- to mesoscopic simulation.

At the transition point, additional meso nodes and meso edges are created. For vehicles entering the meso area, the macroscopic speed specified for the first edge after transition is used. If this edge is very short, significant fluctuations occur that in turn impact vehicle inputs from the microscopic area. As a consequence, there might be unrealistically high wait times on the micro-meso transition edge, as vehicles are not able to leave it.

At the transition point from meso- to microscopic simulation, vehicles are introduced similarly to vehicle inputs. Here, too, you need to ensure that signal heads, desired speed decisions, reduced speed areas, etc. are not too close to the transition point, otherwise they might be ignored by the vehicles.

Use levels to further differentiate between microscopic and mesoscopic areas

If you allocate a section selected for microscopic simulation to a level, Vissim will run a microscopic simulation for this section and level. This means that the microscopic simulation for this section only includes vehicles on the links and connectors that lie on the level allocated to this section.

Example:

A section is selected for the microsimulation and allocated to level 2. This section covers the entire Vissim network: All links on level 2 are included in the microscopic simulation One half of the length of the connectors that connect links on level 1 and level 2 or vice versa are simulated microscopically. The other half mesoscopically, as the transition between microscopic and mesoscopic simulation is located in the middle of these connectors.

Note: Nodes that contain one end of such a connector may not contain any part of the other half, because a node may contain no transition between microscopic and mesoscopic simulation!

Public transport line without PT stop

If a public transport line does not have a PT stop and starts in a microscopically simulated area, its public transport vehicles are inserted into the Vissim network. But they are removed from the Vissim network at the first micro-meso transition point because the public transport lines without PT stops are not simulated mesoscopically.

Public transport vehicles are simulated up to the micro-meso transition point

If the last stop of a public transport line is within a micro range, the public transport vehicles will be simulated up to the next micro meso transition point.

Use relevant evaluations

For a comprehensive analysis of the entire network, use the network performance and link evaluation (Vehicle network performance : Displaying network performance results (vehicles) in result lists), (Showing data from links in lists). You can record data using node evaluation (Evaluating nodes).