It's huge
Especially in Jarabacoa, and the mtn. areas west of S.D.
And, most of the streets are like dirt tracks anyways, so the oceans are your only real boundaries in the DR.
I'd say ATV's are a bit more popular.
Just pop into one of the many motorcycle storefronts - one big one on Winston Churchill a block or so north of 27th, another is one block west of Churchill on G. Mejia Ricart - and ask for more information.