Information Technology Reference
In-Depth Information
The simplest version of the TRANSIMS driving logic consists in fact of the rules de-
scribed above. Most discussed variations, such as different gap acceptance at unprotected
turns, or different
p
noise
, can be changed by global parameters.
The “Roundabout” Solution.
An elegant solution to many of these conflicts is the
use of small roundabouts at intersections [22]. The advantage of roundabouts is that
the high complexity of interfering lanes of standard intersections is decomposed into
smaller sub-units, where in each sub-unit only the conflict between the roundabout and
the incoming lane needs to be resolved.
Alternative Implementation of Graph Dynamics.
The above description assumes that a
street network is given as a graph. It was already said that this yields realistic description
at links, but may become problematic at nodes. The software package VISSIM [23]
therefore dispenses with nodes and connects links via a second type of links, called
connectors. Such connectors start somewhere on a link and end somewhere on a (usually
different) link. There is no need that they start at beginnings or ends of links. Any vehicle
which encounters an outgoing connector somewhere on the link can decide to go there,
or to continue on the link. However, a vehicle need to select one of the connectors
eventually, since there is nowhere to go once the vehicle reaches the end of the link.
Similarly, incoming traffic is modeled via connectors which end somewhere on the
link, not necessarily at its beginning. There need to be rules which resolve conflicts
between incoming vehicles and vehicles which are already on a link.
As a radically different approach, it is possible to dispense with the graph dynamics
completely. The whole transportation system then is overlayed by a CA grid structure,
and vehicles always move within cells. The typical artifacts with off-axis movement
are compensated for by smoothing techniques. CITY-TRAFFIC [24] seems to be using
this technology; we use a similar approach for the simulation of tourists hiking in the
Alps [25].
4
Moving Particles and Moving Agents
There is a stark difference between typical physics particle hopping models and the
transportation models: in transportation, the particles are
intelligent agents
, meaning
that they have individual and potentially complex rules of how they react to the envi-
ronment. This means that in implementations some pointer to these complicated rule
structures needs to be maintained when the particles move. This immediately excludes
the use of single bit coding, since these pointers typically occupy 32 or even 64 bits of
memory.
In consequence, a simple typical vehicle grid in a transportation looks as follows
class veh {
int ID ;
double Speed ;
...
};
veh* Road[200];
// memory allocation for 200 pointers
