One truck parked in the middle of the road to deliver something, or one car blocking the box at an intersection and it’s a standstill.
Also, Highway 101 turns into a residential street with stoplights for several miles.
I've noticed this tendency for vehicles to be stopped in very inconvenient, like right at an intersection instead of a few meters back to allow for people to drive around and properly stop at a stop sign or attempt to cross an intersection at rush hour when it is obvious that there won't be enough space and will block cross traffic, but these are things that we can slowly improve by actually enforcing the laws already on the books or designing streets in a way that nudge against that behavior.
Widening highways doesn't improve throughput.