Not Just Bikes has a great set of videos on Youtube about how road design and driver psychology help to tame speeds much more than simple road signs.
By now the evidence is clear: people will drive at the speeds they feel comfortable at given their environment (sight lines, spaciousness, density, etc.). Just putting a sign on the side of a wide and spacious road is not going to do much to affect driving speeds.
The most interesting one to me was how tree-lined streets encourage slower driving without any speed limit signs, because it feels more like a tunnel and less like an open road.
> tree-lined streets encourage slower driving without any speed limit signs
Somewhere near me (south UK) has a dual carriageway that approaches a roundabout. There is a fence on the side of the road as you approach the roundabout, so that you cannot see what traffic on the adjacent road is doing, forcing you to slow down on the approach, rather than maintaining a higher speed when it appears you might get away with it. Seems to work quite well.
By now the evidence is clear: people will drive at the speeds they feel comfortable at given their environment (sight lines, spaciousness, density, etc.). Just putting a sign on the side of a wide and spacious road is not going to do much to affect driving speeds.
The most interesting one to me was how tree-lined streets encourage slower driving without any speed limit signs, because it feels more like a tunnel and less like an open road.