A method for determining a set speed of an assistance system for controlling a longitudinal movement of a vehicle involves checking a route ahead of the vehicle with a predefined total route length to determine whether, on this total route length, there are areas with a free-flowing traffic state, areas with a slow-moving and/or congested traffic state, and areas with a gridlocked traffic state. A route length of each area is determined. Depending upon these determinations, it is decided whether the set speed is to be defined by the assistance system and to which of multiple predefined speed values the set speed is to be set.