Personally it’s crossing the freeway where I live. My city has about 100,000 people but only six roads cross the freeway, with three more wayyy on the outskirts that are basically detours. There are also only a few pedestrian bridges that cross it, and zero pedestrian tunnels. The way our freeway works is it goes around downtown with the ocean to the south and west, so people live on the outside of the freeway and then commute inwards. This means insane bottlenecks with miles of cars in both directions trying to get to the other side. It doesn’t help that our four freeway entrances are also at some of these tunnels / bridges, which means people who need to get on or off the freeway are also present. In general it’s a shitshow and I’d really like to see a few more bypasses to prevent this congestion in the future.
NYC does a lot of things right, but its subway is very clearly designed to shuffle people in and out of Manhattan. There are many trips one might take within or between Brooklyn and Queens where the fastest path is taking the train into Manhattan and back out again. There’s been a long plan to build a line connecting deeper parts of Brooklyn and Queens - the Interborough Express - but as is usually the case for any infrastructure projects in NYC, it’s moving very slowly and will be ludicrously expensive.
To really put that into perspective, extending the Q line about two miles and building four stations cost roughly the same amount of money as it took London to build the Elizabeth line, covering 73 miles and 41 stops. And of course, this is London, which has a lot of the same issues of being a very old, dense, and developed city.
We should be using the Army Corp of Engineers as a full time federal team of infrastructure builders and dramatically increase it in size, but alas.