We did know in the 20s. We knew in the 30s. We knew in the 40s. We absolutely knew in the 50s (oil industry funded their own studies on this). We knew before we decided to direct billions into a federal interstate highway system that bulldozed countless communities of color and killed many cities' downtowns and sense of connectedness.

I don't see anything positive about being forced to participate in this car-ownership game where 99% of North American cities are designed around car ownership, and if you don't own a car you're screwed. I don't WANT to own a car, I don't want to direct countless thousands of dollars to a car note, car maintenance, gas, etc. I want the freedom to exist without needing to own an absurdly expensive vehicle to get myself around. There's nothing freeing or positive about that unless all you've ever known and all you can imagine is a world in which cities are designed around cars and not people.

Negative effects were immediately noticed. The change in smog was apparent. Road laws rapidly advanced. Road building standards rapidly changed. Congestion was also very much apparent, and the reason behind massive highway building effort that came some thirty years after the car's rise to popularity.

Really these people decades ago had a great grasp on these things. But why did they "fail" and we still have traffic? They didn't fail really, what failed was implementation not planning. Most cities you see with notorious traffic today, chances are the bottlenecks that exist were planned to be relieved by some midcentury road plan that was for whatever reason, not ever built. Comprehensive rapid transit was often also planned, several times over, but not built or at least never to the full scale of those plans. Catalytic converter was also a great success people today probably don't even think about. You can see the mountains again in California's cities thanks to the catalytic converter.

Leaded gas took longer, but I'd say the tailpipe pollution, congestion, and general capacity related issues were well understood.

Nah. We have no means of predicting the long-term effects of LLMs. Major new technologies have always caused effects that were completely unpredictable during the early phases. Any claim that a much better understanding of the world allows for thinking through the effects is pure hubris.