I'm not sure if you live in CA or not, but the majority of places the rule is 150 yards.
State law says firearms must not be discharged within 150 yards of residences, buildings, campsites, occupied areas, recreational areas or domestic livestock, from or across any road. No shooting from a vehicle.
But PC12031 is triggered if you violate county ordinances, state or national park laws, etc The distances given by individual counties can be anything from 50 yards to 300 yard to a mile from a road and everything in between.
For example near lake Havasu in areas that would otherwise be legal to discharge a firearm:
31. You must not discharge or use firearms in California within one
mile of Parker Dam Road. In Arizona, you must not discharge or use
firearms within the Parker Strip Recreation Area.
Riverside and Imperial county prohibit discharge of a firearm within 300 yards of a road (and various other things) for example.
There is 58 counties in California, and they all have a different distance. Violating that distance with a firearm (as defined by the state not the ordinance) would be a violation of state law triggering the statute.
There may be many with 150 yards as the distance, but there is also many with something totally different.
Shooting laws are also changed by zoning, and forest and park boundaries. If you ever get a detailed legal map of an area you will see even in the middle of some desert or forest you get this tiny little box that will a zoned something else, making it a prohibited area even though it otherwise blends right into the area.
Even many national forests have private land, and state or national parks within them, or similar special zoning within portions that change the legality of shooting or possessing firearms.
I know places you can legally shoot in the desert, and travel 50 feet over an imaginary line in the middle of nowhere and be in an illegal area. The only way to even know would be a detailed map and really good navigation skills or a GPS (or a very wide margin of error.)
As far as a road, I know a few areas specifically set up for shooting in various national forests by the forest service in both northern and southern California. They often have a dirt "road" or trail going to them. Sometimes with the shooting actually done after parking on this fire road.
So I could not answer you about what a road is. Even scarier would be like the ordinance I cited earlier in Riverside county that prohibits discharge within 300 yards of a "trail". What is a trail? Any dirt path made by people or a wild animal?
It would probably takes hours of research to be sure what a road is, when it is a road, and when it applies to shooting, and navigating various portions of the penal code and then making sure case law has nothing contrary or more in depth.
Certainly anything marked as a highway or public road would be a road, but further than that?
But I would be fairly comfortable in saying that if it requires a drivers license to drive upon it then it may be a road (but does that make certain "fire roads" on remote mountain ridges roads?) and if it is legal to drive offroad vehicles there then it is probably not a "road" under the law.
So if dirtbikes, ATVs, sand rails, etc without license plates can legally drive the area they are probably not legal "roads". (But I also know some places that are pretty lax with enforcement and let offroad vehicles actually drive on real roads to get to certain places, so just because some are getting away with it...)