  We address the 80-year-old question of whether a freeway with more lanes
results in fewer or more accidents. For finding the optimally safe number of
lanes, in particular, we look at three types of accidents that are prevalent on
urban freeways, namely "following too closely", "driver inattention", and
"unsafe change of lanes". To do so we extend the intelligent driver model (IDM)
to create a microscopic traffic flow model which is capable of producing
accidents. We study the rate of accidents relative to a baseline 2-lane
unidirectional freeway via Monte Carlo simulation. For each simulation instance
we create a starting configuration involving only a few cars over a short
segment of the freeway and simulate the dynamics thereafter. Furthermore, we
look at the number of shoulders present, and show that the presence of
shoulders can positively or negatively affect the accident rate depending on
the type of accident.
