The correct answer to this open question is the following.
Wha the President meant by “ripple effects from building roads or bridges or sewer lines” in view of expansionary fiscal policy was that by increasing government spending, the public projects help solve unemployment and higher incomes buy more goods and services in the private sector, leading to economic growth.
We are talking about an answer President Barak Obama gave during an interview. The President said that the benefits of the expenditure of the federal government on infrastructure projects were well worth it. He commented that "you get a dollar and a half in stimulus because there are ripple effects from building roads or bridges or sewer lines." During the interview, President Obama was referring to the importance of public investment in priority infrastructure projects that benefited the community.