The 85% is the most sensible amount to set a limit at. The safest drivers are those that actually drive a little bit faster than the 85% speed. The most dangerous are both those that drive way above the safest speed and those that travel at way below. The more you criminalise people for travelling at a speed which is illegal according to an arbitrary limit and quite often perfectly safe the less they are going to respect any speed limit.
The more you set limits too low the greater the number of people will ignore them. Previously law abiding drivers are now ignoring quite a few 'silly' limits as authorities have just been lowering them for no good reason in a lot of areas. If you are finding people that used to criticise friends/partners/collegues etc for speeding becoming 'speeders' you know you have gone too far.
If you set the town centre speed limit to 100mph you wouldn't suddenly find everyone driving at 99mph. It can only be up to the driver to manage their risk at any one time on the open road. You cannot absolutely say any speed is always safe or that exceeding a limit by anything other than a wide margin will always be dangerous.
A safe speed for an unlimited motorway would depend on many factors. Weather, traffic levels, visibility, nature of vehicle, condition of driver, condition of road. The design speed of most motorways is in excess of the 70 limit anyway.
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