What can restrict the functionality of a lane departure warning system?

You answered correctly. Lane departure warning systems rely on visible road markings to detect if the vehicle is leaving its lane. If the road surface is dirty or if there are no road markings, the system may not function properly because it cannot reliably see the boundaries of the lane. The absence of guardrails, however, does not affect the system, as it does not use guardrails for lane detection. In general, these systems require clear and visible lane markings to work accurately.