hektik said:
I wonder if you could tell me what the proper position is where the speed limit on a motorway is 50 but there are no roadworks and no other reason that i could see for them being 50...
A speed limit is a speed limit, whether or not you can see any reason for it.
The only speed limits which are enforceable as an actual speeding offence are those properly signed with black number on white background in a red circle (INCLUDING the electronic ones which come up with a red circle on the overhead gantries on the western side of the M25 around London and, recently, on the M42 between the M1 and the M40 junctions in Birmingham (variable speed limit areas).
On motorways the permanent speed limit is the national speed limit (70mph for them). If a fully enforceable temporary speed limit is imposed then signs of the above description will be put up on the approach to, and throughout the limited area, with national speed limit signs (white circles with a black diagonal line through them) again at the end. Any speeding in areas signed like this is just like speeding on any other road with a speed limit.
The other type of temporary speed limit on a motorway is that imposed immediately following an accident or some other incident. These are signed ONLY by the use of the electronic signs alongside or above the carriage ways, with the number spelt out in white lights and amber flashing lights. These are ADVISORY only. Ignoring them cannot be prosecuted as a speeding offence because they are not a properly signed speed limit. Ignoring them COULD, however, be prosecuted as the more serious offence of driving without due care and attention or reasoonable consideration for others. This is unlikely unless an accident is caused or it comes to notice for some other reason (e.g. there is an accident and most traffic has slowed but someone is seen by the officers at the scene racing through regardless).
These advisory limits are only usually used for incidents lasting an hour or two (accidents, etc) or short term roadworks at quieter times (e.g. slow moving verge cleaning operations or overnight works which set up and clear every night). Unfortunately the use of them has fallen a bit into disrepute - with signs left on for hours when there is nothing there, or the wrong ones used. This has changed recently, with the Highways Agency Traffic Patrols being introduced and they should now be far better.
You should always treat the advisory speed limit signs as that - advice to slow down and be more alert. The problem may not be very obvious (e.g. an unlit vehicle which has just had an accident and is stranded in an unlit area of carriageway or some other debris in the carriageway).