A stretch of the Heads of the Valleys road in south Wales is closed in both directions this weekend in the latest phase of the £800m A465 upgrade.
The five-mile (8km) length between Brynmawr, Blaenau Gwent, and Gilwern, Monmouthshire, shut at 20:30 GMT on Friday and reopens on Monday at 06:00.
It is part of a four-year project to turn the road into a dual-carriageway, which will be completed in late 2019.
Drivers will be diverted on the A467 and A4042, via Pontypool.
The closure could affect rugby fans heading to Wales' opening Six Nations game against Scotland in Cardiff on Saturday.
The latest Heads of the Valleys road improvement will cost about £220m as part of a 20-year project - set to be completed in 2020 - for the route that links Swansea and the A40 at Abergavenny.
The Welsh Government described the route as "a crucial artery of our transport network and the principal road link between west Wales and the Midlands".
The upgrade work involves widening the existing road which passes through Clydach Gorge, which is considered one of south Wales' most important environmental and ecologically sensitive areas.
The Welsh Government said the road improvements will improve safety and journey times.