I've seen disputes about how much (can look them up), but as the abstract notes, this was largely a mid-1970s thing, as advanced trauma treatment from the Vietnam War filtered into the US, although the process started in the last '50s: https://en.wikipedia.org/wiki/R_Adams_Cowley

Murder rates aren't the only form of crime going down. Rape, robbery, property crimes are all down massively since the peak in the 1970s. One speculated reason is the removal of lead.