The problem with blanket reductions is that people do not respect them, its fine to have reductions in specific areas, such as black spots ot schools, but a wholesale reduction loses its credibility as it takes no relation to circumstances and conditions, and therefore where the reductions are warranted people take no notice. Additionally in some circumstances saety is actually reduced as drivers resort to more dangerous overtaking when coming up behind soem slower drivers, below.
In rural South Warwickshire, wheree I live, most rural A & B roads have, for no apparent reason, been reduced to 50mph limit from the previous 60. You always get the driver who religiously does 10mph below the limit regardless of circumstances or conditions, with the limit at 60 these drivers were doing in the region of 50 and most following were content to chug along behind them at that speed. Now with the limit at 50 these guys do in the region of 40 and what you have now is a stream of frustrated drivers who now resort to more and more dangerous overtaking in order to get past these people.
Additionally the police have lost more respect because no-one can see why this was done, the previous limit of 60 was adequate, most drivers were then at or below the limit, just had the occasional one who went over the top - and these people still do. Also, strangely, in the first six montsh after the change there was a noticeable increase in the number of camera vans, cant quite work that out myself
Id rather see reductions of limits for rain/bad weather as they do in France for example, but that wont hapopen in here its far too sensible.