Remember 85mph speedometers? That was the maximum indication, regardless of what speed the car could reach. It was supposed to encourage you not to be crazy, at a time when the national speed limit was 55. Certain cars had speedometers that met the letter of the law (no indication past 85) but the needle kept on going into a large blank area, and "enthusiasts" could get British or Australian (in mph) dial plates.

Granted, **most** cars of the time would have been huffing and puffing very hard to reach 85, and very few were even marginally safe to drive at the speed or higher, but there were a few...

And it's not PTC unless it knows ALL of the speed limit (requires wayside communication or much better GPS and backing databases than most cars have), the braking capability of the vehicle, the weather, the friction coefficient of the pavement, and what the other traffic is doing. And can appropriately react to all that. A few "self driving" systems now can get partway there, under favorable conditions, for a little while. Railroads are held to a higher standard.