How should the fall in public debt interest payments be used?

Interest payments on public debt have fallen for governments in all OECD countries. How could they take advantage of this situation? By reducing the total fiscal deficit and therefore the public debt; By increasing the current primary fiscal deficit (excluding interest on the public debt and excluding public investment); By increasing public investment (which is the preferred option if this investment is efficient and if the lower interest rates persist). Among the United States, the United Kingdom, Germany, France, Spain, Italy and Japan, we find that: The total fiscal deficit is being reduced everywhere except Italy; Public investment is not being increased anywhere.