15 feet
ADDITIONAL: Use caution when applying the above answer. That may be the law in some states. Other states (or municipalities) may have different restrictions.
Vehicles other than fire department vehicles using them, should not be parked closer than 15 feet in most jurisdictions. Of course, I actually seen vehicles parked much closer, some directly in front of a hydrant. The owner of one of those vehicle had the misfortune of parking there when a fire broke out; the firefighters smashed his side windows, and ran their hoses through the car. He also got tickets every few minutes for the entire time the vehicle was trapped there; most fires lasting for several hours, I suspect he may still be paying those fines today, several years after the event.
20 feet
In Virginia, the minimum distance away from a fire hydrant you must be parked is 15 feet.
There is no universal answer to this - it varies by jurisldiction and local law. A good safe estimate is at least 15 feet either side of it.
A safe distance, there is no defined limits of this type as driving near parked cars is situational (at best).
Each state in the U.S. can vary on this law. Of the four states I have had a license, three allow no closer than 15 feet. (Washington state, North Carolina, Texas) Oregon lets you park as close as 10 feet to a hydrant.
driver of parked vehicle
If the other vehicle was parked, there was no other driver to have license, insurance or registration. The driver who hit the parked vehicle is at fault and is liable for all damages to the parked vehicle.
As long as the parked vehicle is parked properly and not illegally parked in any manner, then the vehicle that rear-ended the parked car is at fault. Now if the parked car is sitting illegally (such as double parked or parked in a no parking zone, etc.) then the parked car is at fault or even both the parked car AND the car that hits it are BOTH at fault.
Aeroplanes
10 feet, dont park near it unless you want a hoseline run through your car.
The driver that hit the parked vehicle would be at fault.
Such a vehicle is said to be parked.
He was fined for having parked in front of the hydrant. The homeless man asked to be put into jail rather than be fined.