A desired speed decision (DSD) is a location on a link that, when crossed, will modify the desired speed of a vehicle. A vehicle does not accelerate or decelerate prior to encountering a DSD in anticipation of changing its desired speed. Rather, the DSD permanently updates the desired speed of a vehicle until the vehicle leaves the network or crosses a different DSD. Vissim provides a set of default desired speed distributions. New speed distributions can be created using field data. DSDs can be set for specific vehicle classes.