Car following model for mesoscopic simulation
Mesoscopic simulation uses a simplified car following model based on the work of M. Mahut (References). The main difference to car following models used in microscopic simulation is that braking and accelerating are not included. A vehicle changes its speed immediately and has only two statuses:
- The vehicle is driving at maximum speed.
- A temporal distance is kept to the rear end of the preceding vehicle. This temporal distance corresponds to the response time.
The precise formulation of a car following model depends on the meso speed model selected. You may select a meso speed model for each link (Attributes of links):
- Link-based: less detailed. Can therefore lead to an improvement in simulation speed.
- Vehicle-based (default setting): high level of detail. Vehicles attempt to drive at their individual desired speed.
Car following model for the meso speed model Link-based
The car following model uses fixed link speeds:
These include:
xFollower: Front end position of trailing vehicle on current link
xLeader: Front end position of preceding vehicle on current link
VLink: Meso link speed
τFollower: Response time of trailing vehicle
λLeader: Effective vehicle length of preceding vehicle, calculated from the individual vehicle length and the meso standstill distance.
Car following model for the meso speed model Vehicle-based
The car following model uses individual vehicle speeds:
In this formulation, VLink is replaced by VFollower: Each vehicle tries to drive at its individual desired speed VFollower.
Additional bases of calculation
Both car following models use individual vehicle lengths and response times that are based on driving behavior parameters.
By default, the meso speed model is set to Vehicle-based. Vehicles therefore drive at their individual desired speed in free flowing traffic. Desired speed decisions and reduced speed areas result in changes to the vehicle’s desired speed. When used the following limitations apply:
- Reduced speed areas and desired speed decisions apply per lane and extend over the entire meso edge (Mesoscopic node-edge model).
- Multiple reduced speed areas and/or desired speed decisions cannot be used on the same lane within the same meso edge as they are considered to overlap. In this instance Vissim displays a message when the meso graph is created and uses the information of only the object that is furthest downstream.
For the Link-based meso speed model the following applies:
Desired speed decisions and reduced speed areas are ignored. Differences in the desired speed of vehicle classes cannot be modeled. This means that different speed limits on different lanes can only be modeled with the vehicle-based model. However, in a city center network, the Link-based meso speed model might be sufficient. Use of the Link-based meso speed model in most parts of the network can lead to enhanced simulation speed.
Lane selection for lane change
The lane selection for changing lanes of a meso vehicle is influenced by the upstream attribute Lane change distance of connectors and by the downstream attribute Meso Additional Look Ahead Distance (Lane Choice) of the meso edge on which the vehicle is traveling. This enables realistic lane changes, especially when the vehicle must change lanes over several short consecutive edges in order to reach the connector.
When a meso vehicle on a meso edge performs lane selection in the exit node of the edge, the lane preferences of the vehicle on the meso edges are taken into account up to downstream connectors whose Lane change distance extends back into the upstream meso edge. This lane change distance is 200 m by default and can be changed.
With the attribute Meso Additional Look Ahead Distance (Lane Choice) of the meso edge, you can define the distance from the end of the meso edge at which downstream meso edges within the Lane change distance of the connector are considered for lane selection (Attributes of meso edges). Connectors in micro sections are not taken into account. Meso vehicles do not look ahead through micro sections. For a downstream link edge to be considered for look-ahead, it must lie entirely within the look ahead distance.
