The tricky part, in the US at least, is that the "Interstate Highway System" is actually the "Dwight D. Eisenhower National System of Interstate and Defense Highways":
The Interstate Highway System gained a champion in President Dwight D. Eisenhower, who was influenced by his experiences as a young Army officer crossing the country in the 1919 Army Convoy on the Lincoln Highway, the first road across America. Eisenhower gained an appreciation of the Reichsautobahn system, the first "national" implementation of modern Germany's Autobahn network, as a necessary component of a national defense system while he was serving as Supreme Commander of the Allied forces in Europe during World War II. He recognized that the proposed system would also provide key ground transport routes for military supplies and troop deployments in case of an emergency or foreign invasion.
The publication in 1955 of the General Location of National System of Interstate Highways, informally known as the Yellow Book, mapped out what became the Interstate System. Assisting in the planning was Charles Erwin Wilson, who was still head of General Motors when President Eisenhower selected him as Secretary of Defense in January 1953.
The initial funding came partially from the military budget:
The addition of the term "defense" in the act's title was for two reasons: First, some of the original cost was diverted from defense funds. Secondly, most U.S. Air Force bases have a direct link to the system. The purpose was to provide access in order to defend them during an attack. All of these links were in the original plans, although some, such as Wright Patterson were not connected up in the 1950s, but only somewhat later.
The money for the Interstate Highway and Defense Highways was handled in a Highway Trust Fund that paid for 90 percent of highway construction costs with the states required to pay the remaining 10 percent. It was expected that the money would be generated through new taxes on fuel, automobiles, trucks, and tires. As a matter of practice, the federal portion of the cost of the Interstate Highway System has been paid for by taxes on gasoline and diesel fuel.
There aren't many examples where the defense budget is robbed for the public benefit, but the interstate freeway system is one. And as soon as you start talking about the infrastructure that we created and maintain so that we can better mobilize our troops against the red menace, the general public gets that face-scrunchy look and the Libtards start screaming about John Galt'n'shit.