(KEYSTONE, Colo.) — The Colorado Department of Transportation (CDOT) and Colorado State Patrol (CSP) celebrated the 50th anniversary of the Eisenhower Tunnel on Wednesday, March 8.

A key part of Colorado’s highway system located 60 miles west of Denver started construction in 1968, the Eisenhower Bore opened on March 8, 1973 with the other tunnel, Johnson Bore opening in 1979. The project started in May 1943 after the completion of an exploratory tunnel under Loveland Pass.

The construction project was originally supposed to take three years to finish but ended up taking five. 6,000 people worked with heavy machinery for nearly five million hours, their main job was to blast and drill through nearly 8,000 feet of rock and earth. The project cost $110 million and new engineering methods were needed to shore unstable rock so a hole 50 feet high and 45 feet wide could be supported.

Since the project was finished, more than 434 million vehicles have driven through the tunnel during the past 50 years. CDOT and CSP celebrated the milestone on Wednesday. CDOT paused westbound traffic for 50 seconds while a CSP vintage 1970 Fury was the first vehicle to usher in the tunnel’s next half century. According to CDOT, a state patrol vehicle was the first vehicle that entered the tunnel 50 years ago.

According to CDOT, the tunnel saves the public 9.1 miles by not having to travel over Loveland Pass. There are 28 ventilation fans in both tunnels that each move 543,000 cubic feet of air.

CDOT said when the tunnel first opened, some called it the longest bathroom in the country due to the porcelain wall tiles.