I spent the New Years weekend in the LA area (drove from Phoenix) and saw alot of neat places to ride ATV's along the I-10. Not sure if they are legal riding area either. Here's a quick list:

1. Hills along the north side of I-10 near San Bernadino. Looked like a bunch of dirt trails and some hillclimbs
2. Palm Springs - The large sand hill right along the north side of the highway. Looks like sand, can you use paddles? is it legal? There were some rails there and a quad or 2.
3. Palm Springs - The trails behind this large sand hill. Looks like they go on for miles.
4. Palm Springs - The sand area next to the mountains at Hwy 111 about 2 miles off the I-10 intersection. There was an ATV Rental shop right there
5. Indio / Chiriaco Summit area - on the south side along I-10 there were trails that parallel the Hwy.

Thanks.

 

Predfan,
I have wondered the same thing myself. I rode once off the 10 in Beaumont and got busted for being on Department of Fish and Game property without permission. Cost me and my 5 buddies 200 bones a piece for that trip. I'd just give a word of caution to you and others that may venture out in these areas. If there is any sort of sign warning you to stay away, no matter how old or beat up it may look, it is there for a reason.
I have a buddy that works for the city of Indio and he says that he sees cops writing tickets to off roaders around Palm Springs. He says it has to do with the dust that gets blown across the freeway, and lots of people have complained.
Good luck and keep your eye out for those signs! I sure didn't!!