A pike also refers to a long pole. That's basically what was placed across the road as the "barrier" which had to be turned in order for the traveler to pass.To follow through, I looked up turnpike. It's derived from turning and from pike, which is a sharp-pointed weapon. Now I'm left wondering what a toll road has to do with a sharp weapon. I obviously need to "get the point!"
Turnpikes are much older than cars (automobiles), and I suspect it didn't need to be an especially strong barrier if one was only blocking horses (ridden or driven). Horses will naturally stop when you place a barrier in front of them that they don't think they can walk over. They also don't like crossing narrow ditches. It takes a courageous horse to jump some things that a human would simply hop over or duck under.
Since "pike" also refers to a type of fish, I've always thought of the toll-keeper keeping a mounted fish as a barrier. Of course, the really wide roads would need something longer than a pike, maybe a blue marlin.