How would I go about automated mass tagging?

The short answer is "You don't".

Automated edits like this don't add anything to the data, since any users of the data could infer the same information you're adding by using the same rules you are. Mass, automated tagging is also inaccurate, since there are always exceptions to rules, and your code won't spot these.

Even if you could justify the principle of mass tagging using a bot, you'd have to demonstrate your code worked properly and did no harm. By the time you've done all that, you might as well have done the tagging by hand, more accurately and without problems.

It is a bad idea. If the speed limit, or indeed anything else, can be automatically inferred then it should not be tagged explicitly. You will only burden the database (and all its users) with that "massive amount" of tags, plus your tags will occasionally be wrong (since you're only guessing the max speed and not observing it).

You may think that this improves the map but it doesn't. Just like writing good prose - every word you can leave out without changing the essence of what is said will improve the text ;) It improves the map if you go out and add actually surveyed information, not if you write a bot and add tons of tags that do not add information but noise.