Hello Captain
You see, you can put together a sensible posting. - - Lets consider some of the points you make.
Your opening paragraph makes a startling statement, but its needs some clarification. What do you mean by Dangerous? I assume you are referring to the danger of loosing driving control resulting from speed induced instability. In your seventh paragraph you state that it is possible to need to reduce speed to suit road conditions. By recognising that changing conditions will affect the stability of an outfit you are acknowledging that there is danger. If the roads condition makes you believe that you should reduce your speed below the stated speed limit then you have decided that it can be dangerous to exceed the stated speed limit. You are defeated by your own posting.
Drawing on your own insistence on using hard evidence, where is your verified and validated evidence that an outfit will stop quicker than a solo car in all cases.. I recall from other postings on this site that some people find that a car with ABS may have a shorter stopping distance than the solo car, but what about cars without ABS, and those towing trailers without brakes, not to mention the road conditions. My view is that the heavier a moving vehicle is the more energy the brakes need to dissipate to bring it to a halt. Given that drivers do not change their brakes for more powerful brakes when towing and caravan brakes are still simple drums, there is logic to the thought that a bigger rig MAY need more space to pull up safely.
Fortunately we are still in a relatively free world, Shiraz chooses not to exceed 50 on a downhill section and Steve can travel at 55 on the motorways if he chooses, you can travel at 60 if you wish. Lets turn it round and ask to see your hard evidence that it is safe to go down and incline at more than 50 in all vehicles.
Whilst Lutz posts very well argued and supported responses, even I don't believe he is the official spokesman of the French, German and British Governments for decisions on speed limits, so your question is bit unfair.
If you are old enough (and sadly I am) you may remember that during the late 1970' we had an oil crisis, and fuel was rationed. The government decided to impose reduced national speed limits on all roads to help preserve fuel stocks. This shows that safety is not the only consideration when speed limits are set., When speed is a consideration, it is frequently to do with the safety of other road side users, or the fact that the road may pass through a residential area or near a school. In fact speed limits are not set to reflect the capability of the vehicles but exclusively on other factors.
I believe that you are correct when you say that both cars and caravans have improved over the years and it is probable that outfits could attain higher speeds than outfits of say the 1960's, but just because you can do more that the speed limit does not mean you have to, taking your argument further, as all 4X4s can go across poor terrain should the drivers be allowed to drive on all the soft verges or across all fields?
The hardest fact about speed, is that the faster any vehicle goes the bigger the mess and the less survivable when it does crash.
I have a problem with your approach of" I want to be able to do it unless there is hard evidence to say I can't", it is rather selfish and It doesn't recognise that there can be an impact on others.
Under those conditions conflict can arise. In the UK we do have a democracy of some type, and to be able to enjoy the benefits of such a society requires that its members do adhere to certain norms of behaviour and especially tolerance and consideration of others.
In terms of driving, as you say the conditions will dictate the appropriate speed for any given vehicle, and of course the relevant speed limit.
Life is rarely made up of black and white choices , there are so many shades of grey, so consider softening your approach to:
"If its legal and I won't be offending or harming anyone them I may do it"