Performance Improvements for Large-Scale Traffic Simulation in MATSim - Computational Approaches for Urban Environments - page 211

As mentioned above, the messages are not only ordered according to time but
also according to the message type, which is not the case in most event-based
simulations. This is required to solve the situation, in which two events happen
at the same time and a deterministic order needs to be maintained. For example, it
is logical that an agent first has to arrive home before any activity can be performed
there. Therefore, if the arrival at home and the start of an activity there happen at
the same time due to a missing delay in-between, priority needs to be given to the
arrival event for it to be processed first. This is important both for the internal and
external consistency of the traffic simulation, including event handling.

9.3.1.1 Simulation of Transportation Modes Besides Cars

While DEQSim only supports the simulation of cars, a simple and general model
for other transport modes is present in JQueueSim. The model allows to define
a constant travel speed for a new transportation mode. This model has also been
implemented in JDEQSim.
Two other features, which distinguish the DEQSim and QueueSim model, are
described in the following section, as they have been implemented in JDEQSim.

9.3.1.2 Traveling Gaps in a Queue

When the front car in a traffic queue moves, it leaves behind a gap which travels
backwards. Cars behind in the queue have to wait until such a gap reaches them
before they can start driving. Such gaps, traveling backwards as a traffic queue is
dissolving, are implemented in DEQSim (Charypar et al. 2007b ). They have also
been implemented in JDEQSim as this makes the model more realistic.

9.3.1.3 Prevention of Gridlock

Gridlock, which can occur in all of the micro-simulation models, is a problem. For
example, if links are full and there is a circular flow relationship between vehicle
movements on two or more links, it can lead to vehicles waiting for each other
forever. In order to address this issue in DEQSim, if a vehicle waits for a long
time at the front of a link, it is moved to the next link. This introduces a minimum
flow at network links. Furthermore, the space available on a link is also temporarily
modified.
An alternative to this mechanism could be to remove the agent and its vehicles
from the simulation if it does not move after a certain maximum duration at the front
of a road. This mechanism is implemented in JQueueSim but has the disadvantage
that, as the agent is suddenly removed during the simulation, the scoring of the
agent and further processing is stopped. This measure is rather abrupt and can lead