Speed limits usually have been set by data, it's just bad data or badly used data. Like one of the actual ways they determined speed limits was to see how fast people actually drive through an area and then set it so 15%of them are above it.

Of course, much of this was done a half century ago or more. Now most roads have speed limits set by simply choosing one of the 'standard' numbers.

But the real main issue that some studies have shown is poor road design. A road needs to be designed to make the driver adjust to the appropriate speed. A wide road with wide clearance on either side encourages higher speed. A road with trees very close to the road and narrow shoulders encourages you to slow down.

Design roads to encourage the speed you want, a d you'll mostly get it.