Anywhere and everywhere is a bit of a stretch. A 4 lane median separated rural highway is about the safest road you can have... and with wide shoulders it can also accommodate cyclists and pedestrians (though those are rare on rural highways)
I think the biggest thing is that you want traffic moving... not stopped... but also not moving too quickly. And you want a faster transit alternative. The problem with American cities is often even with these traffic calming road diets, you are not offering an alternative. Now the buses have to drive slower as well. And dedicated bus lanes are only feasible in the "bus"iest of areas (see what I did there)
Sure, rural roads are a bit of an exception, as are limited access highways like the Interstate system. You're not going to get much foot traffic along a farm or whatever (you might get recreational bikers, but people aren't going to use it as a primary form of transportation except in more built-up areas). But traffic calming will still save lives in just about any context.
For the most part road diets don't even raise travel times much, anyway; the psychological perception of not being able to floor the gas pedal looms larger than any actual time loss, which tends to be pretty minimal. Oftentimes having roads designed for 40 MPH speeding just means that you spend more time at stoplights. And (as I've said many times before) I know that most people can't just give up their cars, not with our current zoning and transit funding, but there's still plenty of room for improvement on the margins.
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