Information Technology Reference
In-Depth Information
The safety criterion is in fact important in order to maintain laminar traffic [21], an
aspect that should not be forgotten if one has spent considerable effort in designing
rules for stable laminar high flow traffic on single lanes [9].
3
Dynamics on a Graph
A big difference between typical CA models and those used for transportation applica-
tions is that the latter typically operate on a
graph
. A graph consists of nodes and links.
Nodes for transportation applications have ID-numbers and geographical coordinates.
Links connect two nodes, and they have attributes such as speed limit or number of lanes.
Obviously, nodes correspond to intersections and links to the roads connecting them.
Traffic on links can be represented through 2d arrays, with one dimension being the
length and the other one being the number of lanes, and using the driving models from
Sec. 2. The only addition is to include lane changes for plan following, which forms an
additional incentive to change lanes as discussed in Sec. 2. The remaining parts of the
driving logic concern themselves with intersections.
Intersections.
An easy way to deal with intersections is to treat intersections as “black
boxes” without internal dynamics. In this case, the prioritization is handled when vehicles
are about to enter the intersection. There are two important cases: turning movements
which are “protected” by traffic signals, and unprotected turns. These will be discuseed
in the following.
Protected turns are straightforward, since the signal schedule is assumed to take
care of possible conflicts. If vehicles can brake to zero speed in one time step (as is
assumed in most CA models for traffic), then a yellow phase is not needed. The only
other condition for a vehicle to move through an intersection is that there needs to be
space on the outgoing link.
Unprotected turns (yield, stop, merging, etc.) are more advanced. In general, for each
turning movement a list of conflicting lanes needs to be available, which is normally
generated via pre-processing and is part of the network coding. A vehicle that wants
to go through an intersection checks for each conflicting lane if there is a conflicting
vehicle, and only if there is none and if in addition the outgoing link has space, then the
vehicle can move.
The rules for conflicting lanes are normally treated in terms of gap acceptance, very
similar to the safety criterion in lane changing. For example, one can demand that for
each interfering lane, a conflicting vehicle needs to be at least
n × v
cells away, where
n
v
is a small number, and
is the speed of the conflicting vehicle. If the simulation has
n
a time step of 1 sec, then
corresponds to the time gap of the conflicting vehicle in
seconds. In reality, this time gap is of the order of 5 sec; in CA-based simulations, we
found that 3 sec yields more realistic dynamics.
Unexpected Side Effects and Calibration/Validation.
Sometimes, an arbitrary rule, as
plausible as it may be, can have unexpected side effects. For example,
g>n×v
means
that with
v
=0
the gap still needs to be larger than or equal to one. In contrast, with
g ≥ n×v
the turn will be accepted when the gap is zero and the conflicting vehicle is not
