Daily 100,000-plus drivers will use former hard shoulder between junction 34 for Meadowhall/Rotherham and junction 35a for A616 Stocksbridge bypass as part of smart motorway bid to increase capacity (explained here with Highways England video).
Variable speed limits are operating to keep motorists moving along the 10.5 mile stretch while overhead electronic signs are also designed to help prevent queuing and keep drivers informed about their journey ahead while emergency refuge areas provide safe places in case of vehicle breakdowns.
To mark the event here's a timely quiz to test how well you know UK's motorway network.
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Next week speeds will be limited as technology is calibrated. Roadworks and 50mph speed restriction will remain between junctions 32 for M18 and junction 34 while work continues before expected completion next year.
Highways England’s senior project manager Andy Kirk confirmed: "We are investing record Â£15 billion on improving our roads, making journeys easier, safer and quicker.
"Smart motorways are safe way to increase capacity on our roads, proven to cut journey times and wasted hours stuck in traffic".
He added: "This is a great milestone in the scheme with more than 100,000 drivers a day benefiting from four lanes of road ahead of smart motorway full opening next year".