I drive Highway 99 to Sacramento frequently. It is a major thoroughfare running north/south and parallel to Interstate 5 a few miles west. The traffic conditions are frequently very congested, even on weekends and, yes, if you leave the advised space three cars will pile between. That said, I think Californians are terrible at securing loads, big or small. (It should be required in high school to know how to secure loads.) My husband had to dodge an airborne water trough. I had a truck dumping drywall sheets on the highway in front of me that exploded into powder. I was forced to run over a heavy object which dropped from a pickup and got my first dent in the new car. Roadsides are littered with chairs, coolers, shoes, bbqs, file cabinets. The funniest was a truck hauling a trailer load of porta-potties one afternoon on Interstate 880, a very busy stretch south of Oakland, California. A door on a porta-potty was swinging open and the wind unrolled toilet paper, literally TPing the freeway.
[if you leave the advised space three cars will pile between.]