Wait, what? Paris streets are a mess, even after Haussmann. That does not prevent outstanding transit. Even before the latest obsessive aggressive agenda-driven nonsense. The two things that Paris has are (1) inhabitant and workplace density and (2) the will to pursue public transit decade after decade.
After that, there are lots of streets with a single lane of traffic - and buses go through them just fine. And there are lots of too narrow streets but they are isolated here and there - in between streets that are okay for buses. Equally there are lots of very wide avenues for several lanes of traffic and that's fine for pedestrians and transit even though it makes subway stations sometimes very wide. And there are lots of randomly angled streets. There are steep hills. There are rivers and canals and giant mining voids which make building subway tunnels interesting. There are lots of train track rights of way which sometimes block pedestrian traffic but are covered and crossed reasonably often (sometimes still not optimally so). There is snow, ice, floods, hot weather. You name it it's there.
Both of these are essential. "Decade after decade" is essential. Both density and building that infrastructure are impossible to build at the stroke of a pen. So an advantage of Paris compared to San Francisco is that it had decent inhabitant and workplace density from the infancy of public transit when it was private animal pulled carriages.
So what's missing in US cities: decade after decade of density followed by decade after decade of public intent. SF could do it - but for sure you wouldn't see the result tomorrow - but NIMBY so no, SF can't do it. Not before getting rid of NIMBY. LA is much more willing to build anything anywhere so it would have much more of a chance. But it's huge and people often live far from their place of work. Smaller areas with LA would have a better chance. Perhaps Glendale plus the entertainment industry north-south corridor? That might be manageable.
Yet still more "even then", ride Paris transit at rush hours in summer and you won't feel that it's all that good: it's sweaty, packed, and there are lots of timetable incidents. You will regret having chosen rush hour. You car would have been nicely air conditionned and comfortable (but depending on your trajectory would have been stuck in traffic for a while.)
