Seems to me it was relatively safe...running in a dry paved riverbed doesn't really sound like illegal street racing...hell it was almost a real dragstrip except for the safety stuff of course! Better than doin 110 down main street in a 69 roadrunner (wasn't me ...I swear!)
There is a very cool picture of this very event on page 160 of "The American Hot Rod" by Dean Batchelor. Lots of cool cars but none of the Rat rod style we all know and love. For me the coolest ride in the picture is the police motorcycle but I wouldn't kick the 53 Merc out of my garage. The picture is credited to the UCLA Library Hearst newspaper collection. I tried to Google it with no luck. If you have the book (you should!), check it out.