Three weeks of Handy Cross roundabout improvements began on Monday.
The £160,000 upgrade will install a computer system to automatically manage traffic at the roundabout, where the M40 and A404 meet.
Work takes place from 10pm-5.30am using lane closures, and the system may take another four weeks to optimise.
Further down the A404, improvements at Bisham Roundabout, where the A308 meets, have stalled.
Plans to convert it into signalled crossroads were scrapped and it is now back in its “options phase” to decide alternative plans.
Mandy Robson Brown, Bisham Parish Council chairman, said: “We didn’t want the change in the first place, it was the Highways Agency pushing for it.”
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