Object marked with the speed limit, sometimes

BUOY

Think of a floating marker you might see in the water that helps boats know how fast they can go. It’s not a sign on land.

This clue plays on the double meaning of "buoy." While a buoy is a floating marker often used in water, it can also indicate speed limits in certain areas, like no-wake zones. The word "sometimes" hints that not all buoys show speed limits, but some do—making it a clever twist on the usual definition. The answer fits because it’s a literal object that fits the description.