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.
I live in an old market town (UK), so the worst aspect of my town’s planning is the complete lack of any planning. 😀
In all seriousness, I’d say it’s the lack of any major roads into the town. There’s effectively only three roads that go into a town of 35,000 people, all with only a single lane in each direction. This is despite the fact that the town’s major industries are mining and tourism, both of which need decent road infrastructure. One of the roads enters the town with a very sharp 90 degree turn, which lorries nevertheless need to navigate in order to deliver goods to shops, because all the shops are next to that road.
The people that founded the town in the 10th century really didn’t give any thought to what might happen in the following 1000 years!
While I do find it lovely to stumble around twisting alleyways in old cities, I remember being lost in a section of Florence asking “Who in the hell thought this was a good idea?”