Dry pipe systems are usually located in areas subject to freezing, such as a portico, loading dock, etc. They are maintained, by a pump, at a lower pressure than the incoming water supply. (It's isolated from the supply pressure.) When a sprinkler head is activated by heat, the pressure drops even lower, allowing water into the system and to the activated head. Systems are tested at a higher pressure when installed. I am suprised, though, that you found pressure in the system that exceeds that on the supply. Sounds like a bad gauge, or at least a faulty reading. I haven't heard of departments defining residential sprinkler requirements according to proximity to a cul-de-sac, but I know that Fire Prevention Bureaus are pushing for them in residential construction more and more. They are relatively inexpensive to install during construction. If you have one, learn how to shut if off. The fire goes out, or is kept under control, but the amount of water put out by just one head creates a lot of damage.