finally a subject I truly know being a Traffic Officer! (watch me get it wrong)
In the UK the limits are (in MPH)
20 or 30 in built up areas (20 becoming more common now)
40-50 on urban highways
60 is the national speed limit on non-urban roads
70 is the limit on motorways
I think 20 mph is about right in residential areas but 70 is way too low on the motorway. The unofficial limit is 80 anyway in that the cops won't usually pull you if you're within that. I think that should be the official limit with a tolerance up to 85.
for the record I never break the limit intentionally in the city but I always break it if I get the chance on the motorway.
70 on dual carriageways too not just motorways
As the above poster, the National Speed Limit is confusing
On a single carriageway (aka a road which is one continuous surface with no central reservation) the National Speed Limit for CARS* is 60mph
A single carriageway can have multiple lanes, you frequently get three lane single carriageways and infrequently get 4 lane single carriageways on rural UK roads
A Dual Carriageway refers to a road which the two opposing flows of traffic are separated by a central reserve, on which CARS* can do 70mph
You get single lane (one each side of the central reserve) and multi lanes (each side of the reserve), both are 70mph
country roads with no street lighting are unrestricted and default to the national speed limit
all roads with a system of street lighting (defined as no more than 183 m apart) are restricted roads and fall to the restricted national speed of 30mph.
Other speed limits that can be introduced by processing a Traffic Regulation Order, so ;
20mph limits and zones
30mph (on roads which don't have street lighting and thus are already 30mph)
40mph
50mph
60mph
the logic behind setting a speed limit is set out in various government guidance, which roughly follows what is quoted above (20/30 built up urban areas, 40mph semi urban areas, etc)
Whilst rare speed limits can also be raised and recently a lot of UK A class roads were reviewed and the speed limits altered, but the main problems with speed limits in this country is Political involvement!
Politicians like votes and pressurise local authorities to impose speed limits that will gain them votes, for example - the A58 going between Leeds and Wetherby, its a rural road which runs through fields and then comes through villages, villages which are predominantly made up of houses set back from the roads. The government criteria for such roads would be national speed limit for the completely rural roads or 50mph where sever bends/hazards are, then 40mph though the villages.
However if you google it the roads are between 50mph and 30mph despite wide roads with houses set back, they are completely inappropriate speed limits which drivers ignore - its so bad and so apart from standard the Police are exceptionally reluctant to enforce them
This is why their is guidance on speed limits, research undertake shows that if appropriate speed limits are imposed the majority (85%) of motorists will conform to them. However if an in appropriate speed limit is introduced a level of non compliance will occur that the Police cannot practically deal with, and introduces a situation where as motorists are already exceeding the (inappropriate) limit, that they are more likely to use excessive speeds as they are already breaking the law.
I'd hazard a guess (knowing what I do about US roads) that similar guidance is in place, but political pressure occurs that sees inappropriate speed limits introduced
*The national speed limit for large goods vehicles, vehicles with more than a certain number of seats (buses etc) are actually lower than that of cars e.g HGV's can only travel at 56mph as a top speed (56 or 55 it matters not), I believe vehicles with trailers have the same top speeds but you'll often see a nob with a caravan overtaking on a motorway