Suppose the infernal combustion engine had not been invented and we were still travelling with horse and buggy for local traffic. Electric vehicles have not been invented either. If necessary, assume buggies have been made even better than the ones from the 1890s. That is, with lighter materials and decent shock absorbers and whatever other improvements possible.
What would the best roads look like? What would they be paved with? Would they be paved all the way across or would there be an unpaved section in the middle of each lane for the horses? Is there some paving material that’s better than dirt/grass for horses to run on?
What I’m looking for here is the best compromise between the needs of the horses and those of the carriages. Carriages want a hard firm surface for their wheels; horses were not designed by evolution to run on hard surfaces. I expect that too much walking/running on hard surfaces is bad for their hooves/legs. So what’s the best that can be done?
Plastic horseshoes permit a better adherence on hard surfaces and are more resistant than metal ones. So macadam as usual for the roads.
Hmm, didn’t know plastic horseshoes existed. How long have they been a thing?
OK, at the risk of being accused of moving the goalposts, let’s suppose plastic horseshoes have not been invented. Say the plastic industry is not advanced enough to have invented them so far. (Without them thar newfangled horseless carriages, that’s quite likely.) What would be the best roads if horses only had iron shoes?
We would still have some sort of steam engine driving us around. Larger cities just could not handle all the horse crap created by individual ownership of horses. So mass transit systems would have been developed around steam tech.
True, but even in the age of steam locomotives, most travel was by foot or horse, with or without a carriage. The railways didn’t displace the horse and buggy since they can’t put in tracks everywhere. So my question still stands.
Even with plastic shoes, concussion is the problem with horses’ legs and hard surfaces. There are rubber shoes which are better but still not ideal. The best answer to this question is a track or tracks in the center of the road with something forgiving to horse legs and something harder on the outside for buggy wheels. For the horses, you’d need something like hog fuel which also has good drainage. The hog fuel would have to be replaced occasionally as it breaks down. Dirt wouldn’t work because it packs hard when dry and is mud when wet. Buggies do fine on hog fuel, but it would be bumpy and need replacing more often. I can see a road system with the two-track roads as freeways, and hog fuel roads for the smaller ones.
One appeal of hog fuel is that it’s biodegradable. However, if we’re using a lot of rubber in this scenario, recycled rubber pieces can also be used with the hog fuel. I don’t know just how long it lasts. The rubber by itself is kind of a slog to get through, but it would add some bounciness and durability to anything organic.
Source: had horses for over 30 years. This scenario is my dream. 
I’ve thought about this some more 
and realize I’ve forgotten one important thing, although I’m sure there are many. There has to be some kind of transition from the horse track to the buggy track that would minimize huge differences in height. If a horse got fractious and stepped on the edge, the horse could suffer a pretty bad injury, as they could going from softer to hard. Hmm. More thought and a good road engineer are required.
You just gave me an idea. Maybe we need to get Nike to produce some horseshoes. With all the technology they use in their road running shoes that minimize damage to runners’ feet and ankles.