Garmin's Mobile PC can have speed alerts set on. If this alert is on and if the road you are travelling has a speed limit in the map database and if your speed exceeds the limit by, I think, about 10% you will get an alert.
I'm pretty sure there is nothing about warning signs available in Mobile PC or any of the other laptop nav programs I've looked at. What sorts of warning signs are you thinking of?
EDIT: Oops, I think I misunderstood that setting. It looks like you have to have some Garmin safety camera POIs loaded and/or custom speed camera POIs loaded. So it would work like this: IF you are in proximity to a speed camera AND if the map database has a speed limit in the database for the road you're travelling AND if you are exceeding that speed, you'll get an alert.
Won't do anything if there are no speed cameras in proximity.
...ken...