AFAIK it’s just akin to a Nearest Neighbor calculation, i.e. it will stop looking as soon as it hits the first candidate edge.
Altering the search radius also comes with performance penalties. As you can imagine searching the world for some road is more intense than a few sqkm. So, please, don’t use ‘-1’. Also, I can’t imagine any use case where it would actually make sense to consider roads 100 km away from the asked location. Rather gracefully error out informing the user/analysis that there was no road around.
We don’t offer a Nearest service yet unfortunately, but others do, such as OSRM. Then you could look for the nearest road and even select one if there’s multiple. Mixing data/code-bases from different providers comes with its own caveats though.