on osrm-talk there is a discussion
https://www.mail-archive.com/[email protected]/msg01223.html
summary:
in a scenario when finding a route to airport/zoo, nominatim returns (?) centroid of the big feature. snapping the centroid to the nearest road network sometimes returns very wrong route (ending nowhere near the entry).

I guess this is a tagging issue, nominatim should know the "normal entry point" from somewhere. Common cases are big features (airport, zoo, train stations) or even houses/addresses (centroid can be closer to some service road at the back, where there is no door).

is there any existing tagging to solve this? Like putting entrance=main on the polygon of the address so that nominatim takes entrance instead of centroid.