Atlanta has started doing that by greatly widening bridges over the Downtown Connector, so they include parks. Example: https://www.google.com/maps/@33.7768753,-84.3909918,3a,75y,4...

I wonder if that's partly thinking forward to a possible street widening?

This particular example basically connects Georgia Tech's campus across the highway, There was no plan on expanding the road. The lawns actually get a lot of use, students studying/lounging during the week and popular for tailgating on Saturdays.

Several cities in the colder parts of the US and Canada have extensive networks of enclosed pedestrian bridges, such that you could cross downtown without ever going outside.

For example, see Minneapolis https://en.wikipedia.org/wiki/Minneapolis_Skyway_System

Something like this (or rather, the reverse) was Walt Disney's plan for the Experimental Prototype Community of Tomorrow he envisioned: https://youtu.be/UEm-09B0px8?t=15m29s

We do. Big highways without enough pedestrian bridges have a horrible effect on pedestrians and bikes, basically cutting towns and cities in half. I currently work just north of a 6-lane toll road, and it's a reasonable bike commute. I used to work just south of it, and it was a much worse commute, because there are no pedestrian bridges or small quiet roads crossing it, only a few busy arterial roads, full of drivers already in highway mode, trying to dive for an exit ramp, not thinking about pedestrians or bikes.

Baltimore has an interesting network of outdoor walkways above street level connecting the inner harbor, convention center, and a few nearby buildings. I have long thought a city could do this for a much larger part of downtown at reasonable expense.

Crystal city VA on the other hand has an underground mall covering well over a mile including a subway stop. The upside is weather becomes irrelevant, but the downside it's a winding path making the surface faster and presumably it costs much more to build and maintain.

I'll have to check those out when I go to the area in the future.

In SF there are a few spots where active use occurs underneath freeways. Also, in Taipei I saw a huge marketplace for jade and for flowers underneath freeways.

Freeways cause so many ill social and economic effects on neighborhoods, billions are wasted every year in the big cities on dead space. There should be a huge investment toward restoring the urban fabric and bringing out the full value of neighborhoods around these structures.

Minneapolis does this: https://en.wikipedia.org/wiki/Minneapolis_Skyway_System

I was there in early fall, when it wasn't cold enough to make a difference, but we went through them for the novelty. Every time we emerged on the street, we were terribly disoriented.

The I-670 cap in Columbus, OH is a good example. https://commons.wikimedia.org/wiki/File:Cap_at_Union_Station...

They kinda did this in Boston and it was expensive but fantastic.

This is the entire point of projects to put freeways in tunnels.

Perhaps in the future it will be less expensive to drill tunnels! Looking at how rapidly automation is advancing, it is highly likely.

But due to the currently high cost of construction projects, there is a lot of infrastructure that must be augmented rather than replaced.

I run into this in Barcelona: https://goo.gl/maps/4fQfpa7CoPH2

On the other hand, look at something like the Cross Bronx Expressway.

https://goo.gl/maps/RfeVWuS2X5J2

A neighborhood torn apart 60 years ago....is still torn apart.

Do you know if the residential/commercial developments around the tunnel were built at the same time as the roadway? New transportation infrastructure is much easier to integrate into new urban developments, but it would be nice to see something done in existing communities.