Mathematically there is about a x^3.5 (i.e. between third and fourth power) between speed and crash risk. So yes slowing people down should save lives - however, it will come at the cost of increased travel times for goods and services. The best road system is one that has lowest overall operating cost - a study done by MUARC in 2002ish found out that the most efficient freeway speeds were 120 for cars, 100 for trucks; on rural highways 100ish for both vehicle types; and on substandard rural roads to be 80-90. However this study was done in 2002 when fuel was 85 cents a litre; operating costs are a part of the model, so these would change.