This could actually be even lower on the highway as well due to the smart suspension (optional, but common) that lowers the car at highway speeds.
Why are suspensions softer in the US? Is that simply related to the fact that a large slice of the rural market is driving on unpaved roads frequently? I would assume this would be an issue elsewhere in the world also, but perhaps not in Europe.
I don't know that cars in the US necessarily have softer suspension than their European counterparts, but I can attest to the fact that it is not due to the rural population. Much of the rural population will be driving pickup trucks or truck-based SUVs, which have much stiffer suspension than a normal car.
I know that older Cadillac models would have very soft suspension (that's why people refer to them as boats, they take bumps like they were riding waves). I can't attest to that being a factor in any higher incidence of rollovers. If there was in fact a higher percentage of rollover accidents in the US, I would put more money on this being due to the fact that the US really likes truck-based SUVs (Jeep Grand Cherokee, Jeep Wrangler, Chevy Blazer/Tahoe, Toyota 4Runner, etc) with lots of weight really high up. My 1998 4Runner even had a warning sticker saying you can't corner like you would in a car without risk of rollover.
In short, people buy cars they don't know how to drive.
Oh right, I guarantee you this is correct. It's the popularity of high center of gravity cars like SUVs combined with a lack of driving ability. Nice thinking.
Why are suspensions softer in the US? Is that simply related to the fact that a large slice of the rural market is driving on unpaved roads frequently? I would assume this would be an issue elsewhere in the world also, but perhaps not in Europe.