85%

OK, this is going to be just a fun little wank.

Do you know how speed limits are set, in civilized countries? Of course you do.

Traffic authorities place an "informal" speed detector, that measures the number and speeds of vehicles that passes on a given road.

Why did I write "informal"? For a number of reasons.

This detectors sensors are not invisible radars or infra-red beams, but usually  just two tubes laid on the road's mantel, in plain sight.

While their distance is set pretty much accurately, there is not need for it to be certified and checked by external authorities in cases of contested results, as they are not - can not - be used to issue tickets or the like. 

No matter what speed they may read, the sensors are not designed to record the vehicle that was running it - they have no cameras, nor do they have any mean to inform authorities of the fact.

They only read, and store in an internal memory of some type, the speed at which the two tubes are stepped-on by the first axle, and the number of vehicles.

(kind of... pretty sure there are plenty of technical details I do not know enough about).

The fact that these recorders are "informal" is important, as otherwise they would influence the behaviour of drivers on that road, which is what they are intended to record.

Once the recorder has accumulated enough data, this is examined and the speed limit of that road stretch is set as the one that is "respected" by a given percentage of the drivers.

In the U.S. and other countries where this system is used, the percentage chosen is usually 85%.

Long and windy preamble, I know - bear with me.

Seeing things as they go, here in Spain, I would say that the locally used threshold seems to be well-chosen: I have seen few places where the speed limit was ludicrously low (when I used to live and drive in the North of Italy, there were a lot more places with oddily low limits, as they were selected pretty randomly and often lowered in response to a single accident, as a way to "placate the populace").

However, here is the funny part:

Once the vast majority of cars will have adaptive cruise controls able to read speed limits cartels (and maybe check their readings against Google maps or something, just to stay safe; a DMV-MOT service for this would be pretty nice), and most people will use them (limiter function on by default? why not... who wants a speeding ticket if it can be easily avoided?), will this system still have a meaning?

What when - if - autonomous drive will become the normal one?

Probably not (unless the threshold climbs to, say, 95%), yet I can clearly see some authorities take their sweet time recognizing it, and pretty much arbitrarily lowering limits more and more till reaching "arbitrarily absurd" values, and grumbling protests will start.

I forecast funnily annoying times for drivers in, say, 15 years.