Just tested that today.
I put the 5.5m height in truck profile and deliberately went on the road with 4m tunnel (under a bridge to be exact). Just before the passing it says that my vehicle is not capable of passing through selected road and there were two buttons instead of plain "DISMISS" like in other alerts I saw. The buttons were "IGNORE" and "ACCEPT" in terms of accepting or not accepting the warning and planning a completely new road.
Ludo, I'm quite sure that no NG's without additional skins are able to show the alert sign onto the map (like overlay), if thats what you mean. Maybe with some additional skins/ux this is possible.
The alerts on the screenshots I posted come, as I suggest before, as follows.
The data (information) comes from .ftr-files, which have all the additional map data - weight limits, height limits and so on. It is used as additional data by the routing engine to avoid (if possible) roads not permitted for the profiled vehicle.
The visual infromation comes from truck.zip and it consist of additional more descriptive messages and additional signs (the ones on my screens). But I'm quite sure there is no additional information or different algorithms in truck.zip.
Also, I never get turn restrictions with distances (refering the screenshots) before using truck.zip. And I never have such sign if the selected vehicle is CAR, for example.
I'm not a truck dirver, but I drive a large cargo van (panel van?) so sometimes I prefer to avoid small, narrow streets and then I switch to truck-profile. And, of course, as you already mention - one must use the grey matter wisely and frequently ;)
Regadrs