The Friday File: 60 years ago, Eisenhower signed the US Interstate Highway System into law. Expected to take 12 years to complete, it officially took 35. Since 1956, the number of miles traveled/year has grown from 625 billion to 3 trillion. While the 47,662-mile IHS comprises 2.5% of mile lanes, it carries 25% of traffic. At 3,021 miles, I90 is longest; at 18 miles, I97 is shortest. Happy July 4^th!