What is the common 'good'?

In ordinary political discourse, the “common good” refers to those facilities—whether material, cultural or institutional—that the members of a community provide to all members in order to fulfill a relational obligation they all have to care for certain interests that they have in common. Some canonical examples of the common good in a modern liberal democracy include: 
the road system; 
public parks; 
police protection and public safety; 
courts and the judicial system; 
public schools; 
museums and cultural institutions;
 public transportation; 
civil liberties, such as the freedom of speech and the freedom of association; the system of property; 
clean air and clean water; and national defense. The term itself may refer either to the interests that members have in common or to the facilities that serve common interests. For example, people may say, “the new public library will serve the common good” or “the public library is part of the common good”