According to the National Highways website, road users are advised to follow the solid triangle on road signs if heading northbound. The diversion route is as follows:
Exit the M5 northbound at J21.
Join the A370 north/east towards Bristol.
Travel north on the A370 through Congresbury, West Town and Long Ashton.
In Bristol and the A370 junction with the A4, join the A4 westbound.
Follow the A4 westbound towards Avonmouth.
At Avonmouth rejoin the M5 at J18.
If this closure impacts on your planned route, please allow extra journey time. Plan ahead, you may wish to re-route or even delay your journey.