Most expensive infrastructure project (inflation adjusted)

The most expensive infrastructure project in history is the United States Interstate Highway System. The construction of this nationwide freeway network was initiated by the Federal-Aid Highway Act of 1956. The first construction contracts were signed on 2 August 1956, but the system – which encompasses 48,890 miles (78,680 km) of roads – was not formally declared complete until 1992. The total cost of construction at the time was $129 bn, which works out to around $516 billion (adjusted for inflation) in 2024.

The project was launched in 1955, under President Dwight D Eisenhower, with the goal of improving both national defence and civilian transportation. The system connects over 47,000 miles of highways, creating a network that allows for efficient movement of military forces in times of war, and providing American citizens with improved mobility.

The project involved the construction of new routes and the conversion of existing roads into “interstates”, fundamentally transforming travel in the USA. Funded primarily by the federal government through the Federal-Aid Highway Act of 1956, the federal contribution was 90%, while the remaining 10% was covered by state governments. The construction took almost 35 years to complete, making the Interstate Highway System a lasting symbol of post-war economic growth and national defence preparedness.