In any long-standing city, that has grown slowly over time streets tend to have grown organically. There are not nice square corners, unless the city was thrown together on some efficient but frankly dull grid. It doesn't take too much wandering before you are 30 or 40 degrees out from the direction you expect.
I often find this in London, and on emerging from the Underground you're left triangulating using landmarks and streetnames. In short, I want to know which way north is.
We could really help people get around by marking street corners with a north arrow. Initially, I'd go for simple chalk markings for a few tests but a clear, coherent, city-wide symbol would be most powerful.
I imagine we could use something like this (adapted from this handy Public Domain compass rose). We'd have to adapt it for a stencil and find some suitable eco-friendly marking. We might get away with just steam-cleaning it in to a somewhat grubby street:
This is not a wholly new idea, brass and stone markings in pavements have been around for a very long time. The trouble is they are – to my knowledge – typically artisanal, decorative works of beauty. That has a place, but I want to do this city-wide, so we need something that's simple and cheap to replicate.
If we were serious about helping people navigate London on foot, we would bring back some form of useful street maps on hoardings. The old bus shelter maps were mind-bending for navigating London's bus network, but served as very handy local area maps. Better yet, for orientation, lay the maps flat so people have a clear idea of which way the map points.