Mesoscopic and Hybrid Simulation

When the size of the road network being simulated increases, there are normally a number of different routes that a road user can choose between in order to get from A to B. For the simulation, this means that the simulated traffic must be distributed realistically among the available alternatives. With mesoscopic simulation as an additional modelling level in PTV Vissim, users can dynamically assign even mid-sized networks with simulation accelerated by a factor of +/-50 compared to microscopic simulation while at the same time studying the effects of phenomena such as blocking back or traffic light signals on travel times.

Mesoscopic simulation in PTV Vissim

Short computing times are one of the major benefits of mesosimulation. The ability to simulate larger networks with up to 800 districts at high speed is all due to the level of detail. This starts right at the modelling stage: Compared to microscopic models, mesoscopic models have a lower level of detail. This reduction in depth of detail (which is not required for dynamic assignment) significantly decreases the effort involved in modelling and makes it more efficient to work with. Users of additional products from the Vision Traffic Suite experience even greater benefits in terms of efficiency. For example, a network model including demand from PTV Visum or a model created in PTV Vistro for traffic engineering analyses can be imported to PTV Vissim and used for dynamic assignment. It is also possible to use large microscopic models with a high level of detail for dynamic assignment in a mesoscopic simulation.

On the software side, the reduced depth of detail is reflected in the use of a simplified car-following model. In microscopic simulation, vehicle behaviour and interaction are modelled as close to reality as possible, whereas mesoscopic simulation is based on a greatly simplified model. Despite the fact that individual vehicles are still simulated in mesoscopic simulation, their behaviour is based on a car-following model which can be solved very efficiently with an event-based approach. Unlike microscopic simulation, vehicle data is not updated at every time interval, but only at specific times. This might be when a vehicle enters a network link or reaches the end of a link. Another advantage is the fact that mesoscopic simulation provides a convenient way to calibrate networks due to the limited number of parameters. The simulation is more stable with respect to minor network changes which also means that phenomena such as gridlocks, where vehicles block each other’s way, occur less frequently.

Hybrid simulation

PTV Vissim users need not make any compromises when selecting the depth of detail, which means they may combine mesoscopic and microscopic simulations as a hybrid simulation. For example, if users need to go into detail at specific corridors or junctions, they can define sections of the mesoscopic simulation in which all modes of transport and their interactions - including pedestrians and cyclists - will be simulated at a microscopic level. This gives PTV Vissim users a tool which allows them to select the level of detail they need for their specific application, without having to make any compromises.

Hybrid simulation in PTV Vissim

Infographic: Meso + Micro = Hybrid Simulation

Simulation of traffic management measures

Traffic management measures are becoming increasingly important when it comes to intelligently directing road users in urban contexts. Thanks to PTV Vissim's ability to mesoscopically simulate even larger networks, the simulation allows users to investigate the cumulative effects of operational interventions in journeys - for example, by opening the hard shoulder to traffic, introducing variable speed limits, or creating systems that influence road users' choice of route. Hybrid simulation in turn allows for finely granulated evaluations and refinements of planned measures.

Traffic light optimisation in real time

Signal control is the most important tool for inner-city traffic control. Thanks to both the products in the Vision Traffic Suite PTV Balance, the software for traffic-adaptive network control, and PTV Epics, the traffic-adaptive signal control instrument for junctions, as an additional module for PTV Vissim, users can analyse the effects of a traffic-light-coordinated urban network and test out different control logics before implementing them in reality. This is where hybrid simulation again proves itself to be a very useful tool for visualising particular areas and processes in the network in detail by means of a microscopic simulation.