Hmmm and dinosaurs were on the planet way before horses and cars so if they come back they can use the roads, now we need to really worry about the carnivores as that's gonna be some toxic poo.
But if a diplodocus takes a dump on the road I can see that being a problem it may be good for a few thousand roses but the average car aint gonna drive over it But that was the complaint so it's maybe a good thing?
The good news for bikers like rabbitstew above is if you come round a corner and see that in the middle of the road a soft squidgy landing should be imminent - it will be a brown everything moment though.
So in summary - Bring back dinosaurs and let them use the roads the T Rex will eat all the horses and the poo is so large you will never drive over it.
Hope that solves the problem for everyone
Always trying to help I am
EDIT: They will prolly eat the cats and dogs as well so that solves another point raised