Signals and speed limits

The Wiki says "Signal scripts can receive speed limit information and make use of it, but they cannot define a speed limit themselves." The more detailed story is this.

Signals cannot alter the track property speed limit (actually a pair of speed limits, for passenger and freight traffic). This speed limit is what you see in the F3 display.

Currently, signals can query the track property speed limit only when the train is no more than 100m from them. Then, it is too late to change the signal aspect, but time is right to query the train speed (they can do that), and compare these two speeds.

Alternatively, a signal can always compare the train speed against some value defined in the script logic (like 40 km/h for German Hp2).

While the signal cannot modify the F3 display, it can send the train a signal (while it is within 100m). This typically is an "overspeed message" which leads to applying the emergency brakes. It will also be communicated as such in the logfile, to contrast it from overrun signals etc.

Showing dynamic speed limits is not a problem by itself. I.e., you can show 70 km/h for one route and 40 km/h for another and nothing for the straight track. But the information what to show needs to be encoded in the signal script which potentially leads to loads of signal variants.