Title says all here. What is a “road apple”?
A pile of horse dung, so far as I know.
Apparently, a frozen horse turd. That’s all I got, though.
That would be Rio, by Duran Duran…
wait… what?
Oh. road apple.
Horse shit.
That’s it. Doesn’t have to be frozen, though, as another poster says.
An underage beer hall outside of Columbia MO
One assumes the term was coined back when horses were on the public roads a little more often than they are now – although there are both hansom cabs and mounted police in several U.S. cities, so this particular variey of apple is as yet not completely unknown.
–Cliffy
A road apple is a horse turd.
Horse poop consists of multiple round balls, about the size of tennis balls. One is a road apple.
Cow poop is gooier, and is called a cow pie.
Automobile-flattened domestic animals are called “road rugs”. This may be a localism.
American traditional knowledge must not fade into oblivion! 
Should we extend this discussion of thoroughway-culinary terms into “street pizza”, too?
A group of mechanics were sitting around the shop at lunch and Jake was spouting off about Nuclear Engergy. Joe fiinally tired of the pretended ssophistication and said :
Jake, do you know why horse manure comes in round balls called road apples?
Jake replied: Uh, no.
Jake do you know why rabbit manure comes in little round balls?
Jake replied: Uh, no.
Jake do you know why cow manures come in round
Jake replied: Uh, no.
Jake do mean to tell us you know all about Nuclear Engergy and you don’t know s__t1