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That's why in some countries streets with limits faster than 40mph have to be fully grade separated from the rest.

Especially for streets with no limit it's useful to separate them, or people literally fly through the streets.

I doubt that's practical in the US. The country has many miles of state highways (and the older US Routes; US 41 north of Chicago is a popular bypass for a toll road) for which a limited-access road with no at-grade intersections is far too expensive to be worth it, but traffic is light enough that a 55 or 65 mile per hour speed limit is reasonable.

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