My perspective is that people are better at what they're doing when they're paying attention. Give them hints (signs) and shortcuts (straight roads), and their minds wander to something else, until something exceptional happens. There are few if any exceptionally good things that can happen in a car (most revolve around avoiding exceptionally bad things).
In becoming a "native" Boston driver, I have noticed that not all drivers pay equal attention. In other places I have lived, the attention that other drivers paid was not as obvious, because roads were straight and everyone was a "good" (rigid rule obeying) driver most of the time. The roads here are twisty, bumpy, often full of holes, merge in odd manners, have roundabouts, unconventional signs, and generally more difficult to drive. Yet I seem to see fewer road-side accidents on the roads that I drive every day.
Drivers new to the area see narrow lanes (more frequent lane violations), reduced traffic control (fewer clear opportunities to change lanes or turn), and faster drivers as chaotic and aggressive. It is also scary for tourists to cross the street in the midst of this apparent chaos (you'll see natives walking right through). So you might say that Boston has been in this sort of traffic chaos for a rather long time, and it has never been significantly more dangerous in reality (certainly in perception). I can only conclude that drivers who rely on a rigid rule-set have difficulty in unexpected situations, but when the exceptional is unexceptional, they will develop a more sophisticated model.
The net benefit of signs might not be to increase safety, but to allow for lazier, more distracted drivers. Some signs are generally helpful (construction / traffic / drawbridge ahead ), but I would like to see a general reduction. People are smart, and only do what is required. We can tell them not to talk on the phone or "drive distracted" with little effect. Give them a couple of (hopefully temporarily) bad experiences while they are not paying attention, and they will certainly pay more attention all the time.
EDIT: I should also point out that many people refuse to drive in Boston, making the driving population more self-selective than other cities. I don't think that this is a problem, but as society becomes more reliant on driving it becomes one. People who do not drive here can still get around by means of the subway, cabs, or plain walking (Boston isn't geographically large). I can't say the same of most other American cities.