1. A device for longitudinally guiding a motor vehicle, comprising: a sensor system adapted to locate preceding vehicles; a controller adapted to regulate a speed of the vehicle to a setpoint speed, either in a following driving mode as a function of the distance from a preceding vehicle or in a free driving mode; an interface to a navigation system adapted to provide information about a traveled route; and a limiting device adapted to limit the setpoint speed based on the provided information; wherein the limiting device is adapted to calculate a limiting value for the setpoint speed for each possible respective route when a travel route is recognized as being ambiguous, and to select a greatest of the limiting values for limiting the setpoint speed.