The new westbound section of the M8 between Newhouse and Baillieston opened a week ahead of schedule. The eastbound section is due to open next weekend.
Commuters on the M8 and A8 have faced major disruption for the past nine weeks as the finishing touches were put to the new carriageway.
It means the entire stretch between Edinburgh and Glasgow is now motorway.
The works were the latest stage in the Â£500m M8, M73 and M74 motorway improvements project.
Transport Scotland said the new road would cut journey times at peak times by about 20 minutes.
The so-called missing link of Scotland's busiest motorway has opened to traffic.