Why do you think the power to build roads is shared by both state and federal governments?

The power to build roads is shared by both state and federal governments because they both need a route to carry supplies and everyday cars.  If only the federal government had this power, then everyday people would be crowded with government supply vehicles, and would not know how to drive accordingly, since the state government has more of an understanding on how people drive since they are more connected.  When this power is shared, the people and the government get what they need and want.