I heard a city planner talk about why adding a new lane doesn't help, and the term they use is "induced demand."

Basically, people are going to take the route that they consider the most convenient, and that usually comes down to time and effort. Traffic hurts both by taking more time and being more stressful to deal with. When you add a new lane to a road, people think that the traffic will be easier there, so they take that route instead of their normal one. So you're just adding more cars to the traffic that match or exceed the throughput of your new lane, basically putting you back at square one but a few billion dollars more poor.

You've essentially added a single lane one-way road to help ease traffic across the entire city.