Before revision: According to Simon Hakim and Edwin Blackstone, "by 1989, [private] roads comprised just of the 3.8 million miles of streets and roads in the United States and only out of the of the interstate system."
After revision: According to Simon Hakim and Edwin Blackstone, "by 1989, [private] roads comprised just of the 4.1 million miles (6.6 million km) of streets and roads in the United States and only out of the of the interstate system."