I know downtown L.A. REAL well. I used to run a delivery route there for a year and a half. I would say that it is safe to ride in any place as long as there are other people around. In other words, stay away from the produce districts, toy districts, and other places that bring large crowds during the day but deserted at night. Also, don't stop to drink or make phone calls next to parked vans. There are other places that I would say are great to ride (Bunker Hill) but for the most part also avoid places that are near freeway entrances/exits for obvious reasons. Just my two cents. Good luck and carry a spare tube or two!