The mod works crookedly and unclearly

• When the train reaches the speed limit sign, it suddenly slows down
  This shouldn't happen! A train can't do that!

• Signs can be placed anywhere
  Beginners do not understand where to install these signs?
  Make it like railway traffic lights, they show where you can place them and do not allow you to place them far from the rails

The feedback is good and appreciated. Please post in English though. I will delete this post soon.

Changed the message to English

The fast slow down is a feature and not a bug. The signs that can be placed anywhere is also no bug. Its a just a suggestion of you to only allow them to be placed near rail since only there they have any effect.
Keep in mind that I have copied that mod. The original mod was designed like that regarding placement and to change that is probably complicated in code. Or do you know how to code it?
The smoother slow down is the case in the original mod so perhaps for you the original mod is better suited.

You find it here: https://mods.factorio.com/mod/TrainSpeedLimit_018_Port

Again: Your ideas are good. But bring them in the original mod. My mod is just a tweak to have nearly instant slow down.