Many hydrants in Northern areas are dry hydrants. Helps prevent blockage do to freezing of water by having a valve lower in the base. A truck hitting one most likely would cause a release of water unless it really ****** it up. Most of the fires in my area are do to chimney blockage or other fireplace related issues. When you need a lot of water, ice can kill.
Also most of hydrants are screwed with screws that intentionally break after strong impact ( they are weakest point of hydrant) and specially placed valve is closed whe hydrant falls of.