Hello Ray,
I agree with a lot of what you say, but the 85% figure is potentially very misleading. As you concede it is not a rule, it is only a suggested guideline. We are unique in the UK in having this low suggestion, and whilst I agree it is sensible target ratio, I have seen several outfits in trouble that I have little doubt were equal or better than 85%. There is a lot more to towing than just a simple weight ratio.
We must not forget that there are legal limits which relate to the tow vehicles specification produced by the manufacturer, and of course the permitted MGW for outfits on standard driving licences.
If a legal ratio limit were to be set why choose 85%? This figure was suggested by the caravan industry, for caravans, But there are many other types of trailers, some are far more stable than caravans and some less, so perhaps the limit should be different limits for each type, Then the problem is defining which group your trailer fits and overseeing it would be come another opportunity for Government to charge us even more tax to administer the process.
As for braking distances, there has been some evidence that suggests that a well set up car and caravan can stop in a shorter distance than the solo car. This is a rather academic test because it only applies in a straight line. As you rightly say, under turning conditions, the outfit does behave very differently, and making allowances for that does mean increasing stopping distances.
With regard to the number of caravan incidents, the travel reports we get are very very biased. You often hear of road closures because of some unfortunate event, but if a caravan is involved the reporters seem to make a meal out of it. Disproportionate weight is given to incidents involving caravans, but in reality it is probably less than 1% of all road closure incidents.
I have no problem if you wish to keep your outfit entirely within the 85% figure, but that does not make others wrong or less safe if they exceed it provided the vehicle manufacturers specification allows it, and they drive sensibly.