All road signs are instructions of one sort or another.
For example, if you want to go to New York, you need to take the third exit off the roundabout ahead.
While a triangle with a red border indicates a warning and the instruction is, watch out and be prepared for...
I like this one, because I live in a city. Well, on the outskirts of a city.
I guess that's why the road planners put up this warning; there would be no need out in the countryside, where such slow traffic is expected.