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.

They failed because traffic can’t be fixed by adding capacity. The inefficiency of cars will mean you can never build enough roads to keep ahead of consumption.

Traffic gets fixed by getting most people to use some other form of transport and leaving cars to the edge case uses.