In Europe you give way to the car to your right and that's it. In the rare case where there's a car going in every possible direction, you just honk it out or light signal until one goes first.
For example, in the UK, roundabouts will be perhaps signposted as coming up - but when you get there, you just yield to the cars already in it, because that's the base law. In the US, roundabouts are less common, and they have a yield sign at every entrance.
Boston is, however, a deviation. Many roads date to the founding of the city and are much like older European cities in terms of layout, but lack any modern signposting.
If I encountered an intersection with no signage at all, the default expectation would be four-way stop.
The US has the concept, but it's not (in California, at least) the default rule for an uncontrolled intersection. For a regular uncontrolled intersection, it is treated as a four-way yield, only a blind intersection (one with less than 100 foot visibility in any direction in the last 100 feet of approach) is treated as a four-way stop.
Yes, it does. But getting permission to drive is cheap and easy in the US, and many people don't know the rules of the road.
> There are essentially no intersections that are not controlled at all by either specific signs or lights.
There are lots of intersections without signs or lights, IME (admittedly, I haven't driven everywhere in the US.)
ORS 811.275
(1) A person commits the offense of failure to yield the right of way at an uncontrolled intersection if the person is operating a motor vehicle that is approaching an uncontrolled highway intersection and the person does not look out for and give right of way to any driver on the right simultaneously approaching a given point, regardless of which driver first reaches and enters the intersection.
I'd expect they exist in most places in the US.