These have been lifesavers. Highway 101 in South San Jose used to be called "Blood Alley" for lack of effective separators. The biggest reduction in traffic fatalities historically accompanied their adoption.

However the US improvement is 3-5X less than other similar countries. For details on that and links to some data sources, see https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4103211/