The TRANSIMS-1999
microsimulation uses a cellular automata (CA)
technique for representing driving dynamics
(e.g. [25]). The road is divided into cells, each of
a length that a car uses up in a jam - we currently use 7.5 meters.
A cell is either empty, or occupied by exactly one car. Movement
takes place by hopping from one cell to another; different
vehicle speeds are represented by different hopping distances. Using
one second as the time step works well (because of reaction-time
arguments [21]); this implies for example that a
hopping speed of 5 cells per time step corresponds to 135 km/h. This
models ``car following''; the rules for car following in the CA are:
(i) linear acceleration up to maximum speed if no car is ahead;
(ii) if a car is ahead, then adjust velocity so that it is
proportional to the distance between the cars (constant time headway);
(iii) sometimes be randomly slower than what would result from (i) and
(ii).
Lane changing is done as pure sideways movement in a sub-time-step before the forwards movement of the vehicles, i.e. each time-step is subdivided into two sub-time-steps. The first sub-time-step is used for lane changing, while the second sub-time-step is used for forward motion. Lane-changing rules for TRANSIMS are symmetric and consist of two simple elements: Decide that you want to change lanes, and check if there is enough gap to ``get in'' [37]. A ``reason to change lanes'' is either that the other lane is faster, or that the driver wants to make a turn at the end of the link and needs to get into the correct lane. In the latter case, the accepted gap decreases with decreasing distance to the intersection, that is, the driver becomes more and more desperate.
Two other important elements of traffic simulations are signalized turns and unprotected turns. The first of those is modeled by essentially putting a ``virtual'' vehicle of maximum velocity zero at the end of the lane when the traffic light is red, and to remove it when it is green. Unprotected turns get modeled via ``gap acceptance'': There needs to be a large enough gap on the priority street for the car from the non-priority street to accept it [33].
A full description of the TRANSIMS driving logic would go beyond the scope of the present paper. It can be found in Refs. [27, 41].