Runners, no. Very few runners. Fairly busy with cyclists but I wouldn’t call it crowded. I rode to the beach and back yesterday and slow cruised it and only had a couple of “annoyances” that were nothing more than just annoyances. It is about 30 miles end to end and I think virtually completely protected the entire route. You can ride along the beach for some number of miles with a ton of foot traffic if you want (I have done it and would do it again). A lot of people do a “Queen Mary Century” riding from Corona, to the Queen Mary and back.
If you ride the SART, bare in mind NOT to park in Corona, start at another park or at the beach.
You can look at the “flybys” on this ride and get an idea of traffic, I wouldn’t call it bad. Also note, I averaged 100w on my PM for this ride, so even at a super chill pace, you won’t be out there all day. Looking at 500’ climbing for 60 mile round trip.
No advocating for or against this route, just information only.