It is a road sign that is meant to implore drivers to slow down.
In this case, being "Mr. Late" means that you are habitually late. In other words, you are late so often that it may as well be your name.
Being "late Mr." means that you are dead. For example, someone might refer to "the late Mr. Smith" if they were talking about someone named Smith who died recently. (see definition 2a here: living comparatively recently : now deceased)
So, all together, the phrase is advising you that it is better to be late, even if it means you are known as that guy who is always late, than it is to speed and risk getting killed in an accident.
So that you can see it in context, here is a photo of another road sign with a similar expression:
(from the flikcr of henrikj, CC by attribution)