Motorists using the East Lancs Road face a week of overnight closures at the Windle Island junction as part of a major improvement scheme.
St Helens Council has warned drivers to expect four days of the road being shut between 8pm and 6pm.
Motorists are being advised to find alternative routes although there will be signposted diversions in place.
On Monday (March 11) the A580 westbound and Rainford Road will be closed, including Bleak Hill Road. The entrance and exit to Tesco will remain open on Rainford Road.
On Tuesday March 12 Bleakhill Road and Rainford Road will be shut at the junction, although the entrance and exit to Tesco will remain open on the westbound A580.
From Wednesday March 13 until Friday March 15 the eastbound East Lancs Road and the A570 northbound and southbound with Crank Road will be closed. Bleakhill Road and Rainford Road will also be shut at the junction. Shoppers, however, will still be able to get to the supermarket from the westbound A580.
There is likely to be some noise as work is being done to the carriageway and there may be additional lighting, which will be directed away from houses onto the construction area where possible.
Access to properties and businesses will be maintained at all times and firms will be open as usual, the council said.
Pedestrian access will be maintained at all times via the permanent crossing route but there will be a diversion of the existing pathway if needed.
The work is subject to weather conditions.
The £7m investment to improve Windle Island – the oldest purpose designed intercity highway in the UK, which sees around 43,000 vehicles pass through a day – began in August to increase junction capacity, improve pedestrian safety, smooth traffic flow and provide better access in and out of St Helens.
More than half of the scheme is being funded by the Local Growth Fund through the Liverpool City Region Combined Authority’s Single Investment Fund after a successful business case was made that it would address key issues in St Helens and support economic growth in the wider area.