Around 6.8 miles from junction 11a to junction 13 has now been expanded to four lanes.
The work is part of a larger project to redevelop a ten mile section of motorway between junctions 10a and 13.
The smart motorway scheme has taken over a million hours of work, 54,000 tonnes of road surface and 55 miles of cabling.
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Engineers have also used 22,000m2 of concrete and installed 213 new signs.
Highways England project manager Dave Cooke said: "The extra lane between junctions 11a and 13 adds valuable extra capacity which will really help traffic at busy times. It follows the upgrade introduced last summer between junctions 10a and 11a; together this will help improve journeys for millions of people every month."
Transport Minister Andrew Jones added: "The start of full smart motorway running on this section of the M6 will improve journeys for all those who rely on this route.
"Smart motorways are part of our £15 billion road investment strategy. They are just one of the ways in which we are improving the road network, reducing congestion and keeping Britain moving."
(LM/MH)
