That's not true at all. A ton of traffic congestion is caused by people making suboptimal decisions in traffic. Blocking intersections, not merging properly, stooping and going instead of coasting at a set speed, etc. really reduce the speed of traffic.

If you created a self driving only traffic system that guided the cars in an optimal path and speed you could increase traffic throughput by a lot.

But the current generation of self driving cars aren't even close to that task. If anything they will clog traffic even more when they panic and stop in the middle of traffic. That prevents accidents but it also will clog streets during rush hour.

But eventually they may solve traffic jams.