Part of Newport's Fairlee Road, at Snooks Hill, has been shut, with drivers warned to expect 'major disruption'.
Those going to or from the Whippingham/Wootton/Ryde area, via Coppins Bridge, will need to divert along Staplers.
The closure coincides with the planned inspection of Floating Bridge 6, which has put the vessel out of service.
It has led to fears extra traffic could cause 'mayhem' in Newport.
While part of Fairlee Road is shut, lights at the junction of Staplers/top of Snooks Hill will be turned off to improve traffic flow.
The Medina Way Link Road will be open as usual for traffic wanting to enter the dual carriageway (towards Cowes and the West Wight) from the Whippingham/Wootton/ Ryde areas.
Scott Burborough, from Island Roads, said:
“While we have made every effort to keep disruption to a minimum throughout the work, we cannot get away from the fact that Coppins Bridge is the Island’s busiest road interchange and despite our best efforts, some delays to journey times have been inevitable."
The work is the final phase of an Island Roads project to improve traffic lights and crossings at Coppins Bridge.