I've encountered the police during routine traffic stops in both the US and in France. Things could be a lot better here, here are some differences:
In France they always work as a pair, are cordial and while they make requests within their authority, they use a normal tone of voice. In the US, the tone starts out slighly more elevated but will instantly escalate to confrontational if they don't get their way exactly how they want. They exude a mix of paranoia and authoritative over-compensation, as they stand at an angle behind you when you are seated in the driver's seat (because the likelihood of firearm interaction is much higher and because they are usually alone in the US).
The ones in France will try to keep the interaction friendly and if you drop a pun they will smile or groan, wheras in the US they will tense up, afraid of being played. In general, the police force in the US wants you to suck up to them, thank them, worship them and make them feel like heros. Inside, they are afraid, insecure, untrained in psychology matters and most certainly underpaid for the job.
This is why anybody with mental illness or exerting their rights have a higher likelyhood of getting injured or killed by the police in the US. The force here has forgotten WHY they work and are so afraid of losing control of the situation that they will use force where it's not warranted and hurt a lot of people.
As a side-note, I've only had a few interaction with the police in Beijing and I always saw smiles and a relaxed body language. Even though I knew full well they are being very strict about rules, they didn't lord it over people, just kept things moving fast and professionally but nicely.
Finally, while on the topic of travel, both immigration and security agents are way nicer in Europe. Here in the US they are often heard barking orders at people, not smiling and often looking for a reason to flex their authority muscle.