A year after it was first announced, St John's Road in St Helier becomes one-way.
For the next six months, the lower part of St John's Road will be southbound only, towards Cheapside.
The trial, requested by parish officials, is to test if limiting traffic on the road improves safety for pedestrians.
Motorists wanting to go up the hill can use various alternative routes, such as Westmount, Roussel Street, Undercliffe Road and Queen's Road.
The trial will finish on 29 November 2024, and pedestrian access and bus routes will remain unchanged.
The initial plan, announced last June, was for a three-month trial of northbound-only access from Cheapside.
It was delayed twice, then new Infrastructure Minister Andy Jehan asked transport officers to review other options before announcing last month that he was reversing he direction of travel.
He told Channel 103:
"At the bottom of St John's Road, there is a real issue with the size of the road for two-way traffic. I have had a number of people contact me about the challenges they face on almost a daily basis.
"I have been personally to have a look at it. I have made a point of driving up and down the road at busy times rather than using Queen's Road, to see for myself."
The trial begins at 9:15 am on Tuesday (28 May).
The road remains two-way on the wider section, higher up the road.
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