I think 48 road and tram types (24 + 24) is good enough.
24 doesn't exist as a number when you calculate in bits. if you increase 16 your end up with 32. And I think 32 will be good enough for both for now.
And I really don't think people should calculate with any future ground type idea. I mean when will THAT be ready then. Five years from now? This needs to work properly NOW without forcing players to pick and choose again when they'd like to use more. THere's enough bits on the map to use. You only need 2 more to give both a 32 (or 31) option. Once this goes into trunk people will create road types like nuts. Subway, wire, pipeline waterways, whatever. Some people might even try and simulate foot traffic with it or motorcycles or whatever.
Seeing how even NOW you can't load unspooled, docklands and country roads without hitting the limit, it clearly shows that the limit is too low.
There's only three newGRFs around now and you already can't use them all.
Also I highly doubt that people see country roads and such as "eye candy". seeing how the creators intend those to be only accessible for certain vehicles, how stone or cobble road in a town could be blocked for traffic to simulate a shopping street, how some roads might be bus only, how some roads might be heavy truck only or whatever... just eye candy doesn't cut it there. If this is done, it should provide the flexibility so creators can really use this feature. People will 100% load unspooled and docklands at first. Docklands already for the CHIPS and ISR stuff and unspooled or something similar for dirt road, asphalt and highway. and you already can't load those. how do you intend for other creators to start making these if they know no player can load them anyway? 15 is not enough.