The ModelUpdater keeps track of the the number of waiting cars during execution. Initially it
sets the servers to slow mode, spending 10.0+20.0*random() virtual seconds on each car. If
the number of waiting cars exceeds 20, it turns the servers into fast mode, taking only 5.0+
20.0*random() virtual seconds to serve a car. The servers are switched back to slow mode if
the number of waiting cars drops below 10.