It is not common but happens sometimes, even in big infra projects. One example is Wilson Bridge. http://www.washingtonpost.com/blogs/dr-gridlock/wp/2015/03/31

It is not common but happens sometimes, even in big infra projects. One example is Wilson Bridge. http://www.washingtonpost.com/blogs/dr-gridlock/wp/2015/03/31/final-wilson-bridge-reconstruction-comes-in-86-million-under-budget/ http://www.washingtonpost.com/blogs/dr-gridlock/wp/2015/03/3... Also, near where I live, the new highway between Helsinki and Lahti was built under budget and exceeding schedule. (No links in English unfortunately.) So it does happen.