Depending on where you are in the US, there are similar signs here too. Usually just posting a minimum speed limit, not a truck-specific one though. But they're not standard across the entire US, so the rule of the lowest common denominator applies.

And the reason most places in the US don't standardize is because we have too much traffic in populated areas. Despite forcing semi trucks to stick to the slower lanes, some will move to the fast lanes either to pass other slow trucks, or just to make up some lost time on their route. Which keeps all traffic at safe truck-driving speeds anyway.

In a capitalist society, time is money and truck drivers can't afford to be late. A lot of delivery services won't pay the drivers if they show up late, which encourages a lot of poor practices on the public roads.

Yes, it's a system that needs to be changed as well, but without standard unions across the country, businesses can pull shady practices and you can either suck it up and deal with it, or quit and find a new profession. The latter being next to impossible to do with the economy the way it is right now.

Also, another reason the US doesn't standardize is because our country is so extremely massive. Most people don't realize just how huge the US is. Some of our individual states are as big as many European countries. And states have their own rights and laws separate from the federal government. So getting everyone to agree to update public road laws across the entire United States is difficult. A lot of our laws were agreed upon a long time ago and there just haven't been much updates since. So modernizing road laws is a challenge, even with a good administration in charge of the federal government. And we don't have one of those right now.

I see people outside the US asking why Americans don't just protest for better laws, rights, regulations, etc. I mean, we do, but getting us to protest in any meaningful way is like convincing citizens across the entire EU to organize and protest for a single cause. It's next to impossible to organize a protest of that scale, but we'd need that because of how large our country is. There could be a protest in Oregon, but people in California next door might say it's an Oregon issue and not directly affecting them. Getting everyone on board for a singular issue is extremely difficult when we're spaced so far apart. A problem on one side of the country might as well be happening on the other side of the planet as far as most Americans are concerned.

And the more our lives are tied to our pithy income, the harder it is to break away long enough for a meaningful protest. A majority of American's lives would fall apart if they spent a week without working, but the rich 1%ers could go months without breaking a sweat. Outlasting those in power is next to impossible with the way our society is right now.

Couple that with that the fact that we're isolated from the rest of the world and most Americans grow up in that echo chamber about how we're the "greatest country in the world" and you get a lot of passive citizens who don't address issues until they are directly affecting them. Which is too late by that point. America is just in a bad place geographically and politically to organize its citizens for better reform.

Sorry that this devolved into political discourse, but that's kind of the root behind a lot of our unchanging and ineffective laws right now.