Traffic doesn't exist because drivers are selfish. Congestion is a fundamental property of the dynamics of the system. As the number of cars per unit area goes up, the speed of those cars goes down, The road's capacity is a product of these two numbers and there is an inflection point at (iirc) 40% car density. If the road's capacity drops, then the traffic that feeds into it will be greater than the traffic that flows out of it, further increasing the density, and dropping the capacity.
The maximum (practical) speed depends on the cars per unit area, but drivers can voluntarily drive slower than this. By looking ahead at the conditions down the road, such as traffic jams and red lights, you will often notice situations where there are big benefits from slowing down more than you are forced to.
Still, a lot of people don't do this, and it makes life harder for those who do. I'm amazed by how people will race to be the first one stopped at a red light.
Doing a greedy optimization for maximum instantaneous speed can reduce the overall traffic speed, and sometimes even the average speed of the driver in question, increasing their own trip time. But the greedy optimization is still common to see.
