It's not arbitrary. In Virginia that's a guaranteed reckless driving charge.

Texas: we set the speed limit at 85.

Meanwhile Germany: that's the target speed you should aim for, but if you want to drive faster, you absolutely can.

Not anymore. It got moved to 85 because speed limit on interstates moved to 70.

How do one know this? I don't know where to get this information and whether to trust it.