There are a few river trails that are totally empty on weekdays, and they go from the beach to 20-30miles inland. Not a single stop sign or stop light. I've not been at a redlight at any time over the last 1000 miles....These days I'm either on a deserted MUP or riding mount baldy.
Maybe Orange County is better in this regard. In San Diego, there are, in fact, almost no flat trails without stop signs or stop lights. I've been meaning to do a new TT lactate threshold test for a while, and the biggest thing stopping me is that I have to drive half an hour one way (preferably at 6 AM) to a place where the test could be done. There's 101 through Camp Pendleton, there's San Luis Rey River Trail (also near Camp Pendleton, flat but with annoying "dips" when crossing city streets); there's Fiesta Island (flat circle, no stop signs, but with persistent 10 mph westerly wind every day from 9 AM to 9 PM, so it might as well be considered a slope); there's 56 bike path, not very flat and with stops every 3 miles; there's Ocean Beach Bike Path (flat, 3 miles long). As far as I know, that's the complete list of flat trails of any notable uninterrupted length between downtown San Diego and south Orange County.
I have no problem finding hilly rides without red lights (there's a route that I rode on a couple of occasions, that starts at my house and goes for 16 miles without a single red light and ~4 stop lights, all 4 of which can be safely blown because there's absolutely no traffic at any of them); but flat rides, those are hard to find.