Well. If the other property owners have a right-of-way across speed-bump-guy's property, he is not permitted to obstruct the right-of-way with a gate, no matter what his lawyer says. Unfortunately, the only way that this can be enforced is by going to court.
Speed bumps are probably not exactly obstructions, though. I'm not sure he can be prevented from putting in speed bumps which don't obstruct the right-of-way. On the other hand, he can't prevent others from removing them.
Forming a statutory road association might help, if the speed-bump-guy decides to respect the authority of the association. If he doesn't, I really don't know if the association has the exclusive legal right to maintain the road. I think it might, given that the statute states, in 23 MRSA 3102:
The commissioner or board chosen under section 3101, with respect to the private road, private way or bridge, has the powers of a road commissioner.
That is, the association's road commission has the same powers as a municipal road commissioner, whose powers are defined in 23 MRSA 2701:
The road commissioner, under the direction of a majority of the selectmen, shall have charge of the repairs of all highways and bridges within the towns and shall have authority to employ the necessary personnel and equipment and purchase material for the repair of highways and bridges.
So, the association might assert to speed-bump-guy that it has sole authority to maintain the road, and is willing to go to court to enforce it. Whether a court would agree, I do not know.