The new M4 underpass at Chieveley in Berkshire, is to be officially opened by the transport minister.
It took 18 months and �38.5m to complete the changes to the dual carriageway, which is the main route between Southampton and Oxford.
It connects the A34 at the Newbury Bypass to the A34 at Priors Court Road.
Regular tailbacks around Junction 13, led to the route being described as "one of the worst bottlenecks in the country".
Drivers on the southbound carriageway will be restricted to travelling at 40mph because some work has still to be completed.
The changes should mean quicker journeys for the 90,000 motorists who use the junction daily.
One-Minute World News
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