I don't speak French per se, but from what I can parse -- you are asking about speeding and enforcement?

Enforcement depends on (a) the state and county/city, and (b) who has jurisdiction over the road.

Some roads will be state police, some will be local police or sheriffs. Some allow for or require use of radar vans or red light cameras. Some do almost nothing except in areas with a lot of crashes.

Construction zones and school zones will see increased penalties if there is a crash or if you are caught, though enforcement is a guessing game there as well.

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It makes more sense when you realize that the political power structure is inverted in the US. There are almost no national standards for anything, and when there are national standards the enforcement falls in production or construction, not in day-to-day enforcement.

For example: the color and location of lights on a car are standardized, but that's not something your average person does. The enforcement for that is done through the manufacturers and factories, not the customer.

All states have a "drinking age" for alcohol of 21, but that is not due to a national law. It is because Congress passed a rule that federal infrastructure money can not be distributed to locations where drinking age is below 21. If someone who is 21 drinks wine, the federal government has no power to take the person to court. Note: you can consume alcohol at any age, but it can't be sold to you at a store or restaurant until you are 21; this is to make it more difficult for 'new' drinkers to get drunk and then try to drive home

If a state applies for federal funding for a local street project, that street has to meet federal design standards. If the state wants to build a road outside of those standards, they can - there is no legal barrier, but they then have to find their own money for the "non-standard" road.

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Laws and enforcement are entirely up to each state, county, and local town or city. Some do well, some don't care.

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This is true for everything in the US except for a dozen-ish things specifically listed in the Constitution and early documents forming the nation. States handle marriage, states license drivers, states each set their own standards for licensed professions such as teachers or nurses, states run their own elections, states run their own schools, build their own roads, register their residents property and vehicles. States also collect their own taxes which are not subject to federal control or use.

The Federal Government only has explicit power regarding: currency/money, international trade, war and peace, and general human / resident rights (eg. right to a trial, right to vote, etc), and a loose "multi-state" power that allows the federal government to develop shared standards for things that cross all states (such as the lights on a car which I mentioned). States set their own laws and enforcement, while the Federal government only pursues crimes that involve many states (eg. organized crime) or if a state requests assistance with a major issue.

States even maintained their own militaries which were deployed under a federal coordination up until about WWI; and states still maintain their own militias called the "Guard" who are under the control of each governor. The modern centralized military we have today is less than 100 years old.