Model description

Scope

Fosim is a microscopic simulation model, that is, traffic operations are modelled through the simulation of the behaviour of individual drivers.

The simulation model was specifically developed to study traffic operations on Dutch motorways. Several road geometry characteristics can be specified, just as a number of local traffic measures (such as, prohibiting overtaking by trucks) and traffic characteristics. The model is mainly aimed at corridors, but to a certain extent it can also cope with route choice situations. In those cases, the splitting fractions have to be specified in the input.

When a simulation is run, Fosim determines the way drivers move on the specified road sections. The resulting traffic operations can be examined during the simulation run and analysed in detail afterwards, both at an aggregated level (for instance, speed and flow measured at cross sections) and at a microscopic level.

Because Fosim allows the user a large degree of control over both input and output, the model is suited for a broad range of traffic studies. For instance, Fosim was used for academic research into dynamic multi-user class traffic assignment modelling, and travel time prediction with neural networks. In most cases, however, Fosim is used in road design.

FOSIM is property of Rijkswaterstaat part of the Ministery of Infrastructure and Water Management.

Main principles

Dependent on the given road section Fosim creates a number of origins and destinations. When a simulation run is started, vehicles are generated at the origins according to the flow rates given in the input. Each generated vehicle receives a destination and certain behavioural properties that remain constant during the vehicles' presence in the simulation.

After a vehicle is generated and enters the motorway, its actions are determined for each time step (in Fosim time is divided into time steps of 0.5 s). These actions depend on both road characteristics and surrounding traffic.

Drivers are influenced by other traffic when they have to adapt their speeds. First, they check if they can overtake a slower vehicle and if they cannot, they follow their leader. Fosim calculates both the acceleration needed to avoid accidents and to follow at a comfortable distance.

Road geometry has considerable impact on traffic operations, especially at so called discontinuities, such as, on ramps, weaving sections, lane drops, et cetera. The expected lane changing behaviour at such discontinuities is an explicit part of the input for Fosim. This provides users with a large degree of control which is useful because it enables them to take (expected) local conditions into account.

Apart from geometry, local traffic measures may also be relevant. In Fosim it is possible to take into account:

• the impact of traffic controllers,
• overtaking prohibitions for trucks,
• speed limits,
• ‘speed suppression’. Speed suppression locally decreases speeds to simulate, to a certain extent, roadway characteristics that Fosim does not explicitly distinguish, for instance, small horizontal curves.

Validity

Fosim is calibrated for motorways that conform to the Dutch Design Guidelines for Motorways. The validity of the model has been examined several times for a large variety of motorway segments (see the Applications section). Consequently, when a simulation study is carried out for the types of motorway segments the model was calibrated for, it is not necessary to calibrate the model.

The degree by which Fosim is considered valid, is illustrated by the fact that the model was used to estimate capacities for symmetrical and asymmetrical weaving sections for the Dutch Capacity Manual for Motorways.