They can ask. You don't have to answer/compily. That's the important distinction

They can also do things that are illegal and then lie about them after if needed. Or just make up some false probable cause to warrant a search.

One ought to be able to consult a lawyer or ombudsman during these attacks on your civil rights.

You haven't committed any crime. Who's supposed to be serving who here?

An ambiguous crime. The limits are clearly posted, but enforcement is highly inconsistent. Driving the speed limit has probably become indicative of old age or an attempt to avoid police interaction. Try strictly driving the posted limit for a while and you will find it socially unacceptable. While most traffic stops are rightly due to excessive violation, some are an excuse to investigate. A stop for speeding makes a great cover should the motorist complain. If they were driving slower than the social norm, or even the cars around them, there is no hard evidence to support their case.

Traffic violations are mostly civil matters, not criminal matters.

Right, and that's why you're not entitled to a jury trial for your speeding ticket. Speeding, running a light, etc., are not criminal offenses (except in extreme cases of reckless driving, where you may be on the hook for "reckless endangerment" or some such)

They can't lie about them afterwards, but they can lie to you during interrogation. E.g. "your friend Joe already told us you did <whatever crime> so you might as well admit it." even if that's not true.

You 'give up' a lot of your rights simply by using public roads.

you mean by operating a vehicle on a public road. They don't have the right to breathalyze you if you're walking home.

If you appear to be intoxicated (staggering about, etc) you can be arrested for public intoxication, though you are not driving. Not sure whether they can compel a breathalyzer without a warrant in that case though.

Where I live they do (its not US).