From comments at mansizedtarget.com
The effect of medical advances on the homicide rate is quantified in this article: http://www.wku.edu/~james.kanan/Murder%20and%20Medicine.pdf
The methodology is obviously not perfect, but this does give at least a good first cut.
Figure 1 in that document is where the gold is, showing an approximately 3.5-4x drop in lethality from 1960 to 2000. The drop off is mostly linear, with a steeper drop immediately after Vietnam.
Figure 2 demonstrates that this improvement is largely in lockstep with automobile related mortality.
Figure 3 shows the same roll off, but with higher lethality rates in areas with less in the way of medical resources, supporting the claim that it is medical improvements largely driving this reduction in the lethality of aggravated assault.