> Speed-limits typically remain grandfathered in at their original value

That depends on where you are. In Texas, state highway speed limits are determined though a traffic study[1]. The monitor traffic for a while, then set the limit to the 85th percentile.

People can use this to get out of speeding tickets. If you find that it's been a long time since a speed study was done on the road you were on, the judge might throw the ticket out.

There are some hard limits though. For example, the maximum speed limit that can be set on a road is 85 mph.

[1]: https://www.txdot.gov/safety/driving-laws/speed-limits/speed...