In Crystal City VA they have a significant underground walkway / mall that spans ~15 city blocks, and connects to the DC Metro (subway system).
Between the two options I prefer the underground walkway, but either option completely change how long it takes to get around. Rather than trying to accommodate both cars and pedestrians if you just separate the two both benefit. Cars can make turns easier and avoid accidents, people don't need to stop at intersections.
Look around the world. Desirable places strongly correlate to people walking at street level, because this allows people to pass shops, cafes, etc. and easily go in. It's the core of city life.
Crystal City is one of the least nice places close to downtown DC. Above ground it has huge streets, little street life and few people walking around. Compare that with Dupont Circle, where the city is full of life (this is where I work).
Raised or underground walkways are a great way to kill a city. Skybridges are a very 1970s idea and they are being torn down. Where I live, Downtown Silver Spring (not far from Crystal City), sky bridges are expressly forbidden in the CBD.
There are some areas way up North (upper Midwest, Canada) where there are underground walkways due to the weather in the winter. Most places, including the DC area, are fine in the winter.
There is a lot of writing about Skybridges out there. Here is a start: http://greatergreaterwashington.org/post/654/skybridges-dont...
I agree, most Skybridges are terrible. But, it's a density and network effect question, when you can walk 6 blocks without going back to street level AND the shop you want to get to is also up/down there AND you take your elevator / escalator to that level then it works. The Crystal City underground suffers from making the underground path a lot longer so you can often get someplace faster at street level. However, I have both lived and worked in the area and used it all the time.
PS: Dupont Circle greatly benefits from being part of a much larger city, it is walk-able with a well connected subway system. However, it's a low density area and driving around there sucks. If live in that area or take the metro it's great, but you can't sustain that when building get taller and people still want to drive.
Paris is largely mid-rise buildings or lower. Paris is denser than NYC and not that far behind Manhattan in terms of density (54,000 people per sq mi in Paris versus 71,000 for Manhattan and 27,000 for NYC as a whole). DC density could be greatly raised without taller buildings by filling in missing teeth (abandoned lots of unused buildings on otherwise built streets) and by urbanizing the large swaths of the City that are suburban. Narrowing some streets to allow for wider buildings would also help.
Ultimately, I think we'll have to agree to disagree. I don't think elevators or escalators really belong in an urban setting. I do agree that being able to get to a store that you know you want to get to via underground tunnels can have its benefits. But one of the things that I like about living in an urban area are all my unplanned walks, where I walk around, check out the shops, grab a coffee, maybe go into a few stores. A lot of urban life is unplanned and when we remove people from streets, we begin to lose that.
Paris is denser than just about any US city and it is largely low and mid rise.
Faster cars would not benefit the core users of cities -- people on foot.
However, autonomous cars should be much safer as they have senors to avoid collisions. Car drivers often come onto sidewalks to avoid hitting cars. So yes, to avoid a minor car accident, car drivers kill people on sidewalks.
The biggest benefit to autonomous cars will be that we will go from 40,000 people dying a year to almost none. That's a huge win.
And if that's all we get, wouldn't that be awesome?