Upgrading 32 miles of M4 to a smart (?) motorway has taken 4 years - simplistically a few signs and road markings on the hard shoulder.
If you look at what they're actually doing there's a lot more to it than you suggest.
The M1 between Milton Keynes and Northampton is being similarly upgraded. Although they've avoided having to purchase new land the carriageway is being widened to a degree. A great deal of work has been done on embankments to dig them out and stabilise. There's also a lot of groundwork involved in constructing and stabilising the emergency refuges.
The signage requires infrastructure including overhead gantries, wiring, data provision etc as do the CCTV cameras. Some sections have what looks like extensive infra red floodlighting.
IIRC some parts of the eastern section of the M4 were originally built as two lanes widened to three in the seventies. Bridges in these areas had insufficient span for all lane running and required replacement.
And all of this while, for the most part, keeping the road running.
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