There are many off-road vehicle opportunities in California, but first you must make sure that your vehicle is registered. All vehicles operated on public lands in California must be registered, either with a street-legal registration or a off-highway vehicle registration. Registered off-highway vehicles receive either a red sticker or a green sticker, with red sticker vehicles only allowed to operate October 1 to May 31.
Many dirt roads are open to registered OHVs, but there are a few exceptions. For example, dirt roads in Death Valley National Park are open only to street-legal vehicles. Also, in many National Forests, some "maintained" dirt roads require street-legal registration and a license. Always check before-hand.
Helmets, of course, are required, and other safety equipment, such as proper clothing, boots, and sunglasses are recommended.
Open Areas are open to all vehicle travel, including cross-country travel.
Limited Use Areas are a gray area as the term is somewhat ambiguous. Occasionally travel on all existing, well-defined roads is allowed, but many times travel is restricted to only signed, marked routes. If it is the latter, be very cautious or you could find yourself with a ticket.
Wildernesses do not allow motorized travel. In many instances, such as National Forest wildernesses, all mechanized (i.e. bicycles and motor vehicles) are prohibited.
Prairie City State Vehicular Recreation Area:
Prairie City is a great park specifically for using off-road vehicles. They have numerous trails and tracks for karts, ATV, Moto Cross, rock crawling and trucks.
13300 White Rock Road Rancho Cordova CA
March 8 am - 6 pm
April 8 am - 7 pm
May- Aug 8 am - 8 pm
Sept 8 am - 7 pm
Oct 8 am - 6 pm
Nov-Feb 8 am - 5 pm
Miami Motorcycle Trails (1), 5 miles north of Oakhurst near southern entrance of Yosemite. The trails are open in winter. Beautiful forest scenery; exciting trails with many roots and ruts. It's easy to get lost in as the forest is thick. There are 60 miles of trails. Mainly for motorcycles and ATVs, Trails are marked and are rated from easy to difficult. Dirt roads are not all that challenging and wouldn't be very exciting for 4wd enthusiasts. There are two staging areas, but there are many other good camp sites in the area. Free. Operated by NFS.
Hollister Hills SVRA, (2) 5 miles south of Hollister (California). Open all year round. Small area, but many short easy trails and hill climbs. Five motorcycle tracks, one ATV track, and and a 4wd practice area. There are two different sections -- one for motorcycles and ATVs, one for 4wd vehicles. Day use $4, camping $6. Operated by State of California. 
Spangler Hills Open Area (3), just south and southwest of Ridgecrest (California). Open all year round, winter is the best time to visit as temperatures are very high in summer. Huge, highly recommended desert open area with some challenging areas of sand washes and rocky mountains and some areas of nice whoops and hill climbs. Open camping available anywhere. All types of vehicles. Free. Operated by BLM. 
Hungry Valley SVRA (4) just east of Gorman, about an hour and a half north of Los Angeles. Open all year round; can get quite hot in summer and snow can close some areas in winter. Many miles of trails and some open areas. Trails of all difficulty levels. Camping and campground and open camping in part of the park. All types of vehicles. Day use $4, camping $6. Operated by State of California. 
El Mirage Dry Lake Open Area (5) about 20 minutes northeast of Victorville or 40 minutes east of Lancaster. Open all year round; very, very hot in summer. Good sized desert open area. Many sandy trails and hill climbs, and a huge dry lake for novices. Open camping anywhere. All types of vehicles. $15 per day dawn to dusk, $30 for a seven day pass or $90 for an annual pass good from Oct 1 to Sept 30. Operated by BLM. 
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