Others have cited the removal of the last traffic light on I-90 in Idaho in 1991 as the Interstate's completion day. In my opinion, the decertifying of US 66 is what focused the public's attention on the ascendance of Interstates - Chicago to Los Angeles without a stoplight. In comparison, US 99 was replaced by I-5. A single highway number linking those two cites had been a priority since US route planning began in 1925. The single number connection became less important over time.
Businesses flourish where they had once withered in an Interstate's shadow.
Interstate highways are wonderful for getting you some where fast, from one big city to another one without all the small towns and stoplights in between. The US highway system can still take you some place , to where someone lives, works or plays. One of the goals of this site is to show where these highways go and have gone. Maybe you will recall road trips from your past, or be inspired to take another one. Maybe you will learn more about the history of your hometown.