I know I am not controlling for road conditions, car conditions, traffic signals conditions and culture of respect to traffic laws, but... in Brazil, IIRC, the lanes are 10 feet (3 meters) and we are world champions for road fatalities per vehicle-km (https://en.wikipedia.org/wiki/List_of_countries_by_traffic-r...)
It does not disproof the OP, but it is good evidence that lanes width is not the most important factor. But I honestly don't know if all other more important factors are fairly good in the USA.
edit: I was checking brazilian regulations and it lane width is defined on a rage from ~8 to 12 feet, depending on the "class" of the road (from small, unused roads in small towns, passing through regular traffic heavy urban roads to interestual highways with heavy trucks).
Even 10 feet sounds ridiculously generous by UK standards! Only on motorways would you normally find 12 foot lanes (and even then, they're not always so wide).
I suspect many unfamiliar American drivers would be horrified by UK roads and wouldn't consider them "safe" - yet accidents and road deaths, per mile driven, are significantly lower.
As an American who got to drive over central Ireland on a vacation and my wife spent half of the time covering her eyes as big farm trucks passed by with just an inch or two of space on my side and an ancient looking stone wall just inches from her side. We weren't even in a big vehicle, it was just a tiny Ford.
It certainly didn't feel any safer, and I couldn't help but to notice all of the sideswipe marks on various vehicles when we drove through towns.
Sounds like parts of my journey to and from work: one side is a drainage ditch backed by a rough granite cliff rising vertically, the other side a short drop into the fjord (short drop because I live on the east coast of Norway not the west).
It does not disproof the OP, but it is good evidence that lanes width is not the most important factor. But I honestly don't know if all other more important factors are fairly good in the USA.
edit: I was checking brazilian regulations and it lane width is defined on a rage from ~8 to 12 feet, depending on the "class" of the road (from small, unused roads in small towns, passing through regular traffic heavy urban roads to interestual highways with heavy trucks).