Drivers are warned of disruption as a key route into - and out of - Newport turns one-way.
Downend Road is one-way from Robin Hill roundabout to its junction with Downs Road. It is one-way from north to south.
Island Roads says it is to allow for improvement works to the footway and kerbing.
The road will be one-way between 9.30am and 4pm until the end of the week at the earliest.
A spokesperson said:
"We have scheduled the work outside the holiday season and when Robin Hill is closed.
"We intend to compete the improvement and re-open the road to two-way traffic by the end of next week at the latest and sooner if possible.
"It will help reduce the likely duration of this scheme if motorists – as the vast majority do – observe the current daytime one-way restriction."