The M25 in Hertfordshire came to a standstill on Sunday (14 July) after a “ridge” formed across the carriageway.
One possible cause that is being looked into is the heat – with the UK experiencing temperatures as high as 32C (89.6F) over the weekend.
The damage occurred near junction 23 on the clockwise section of the motorway. It was closed overnight between 5pm on Sunday and 5.30am on Monday morning, while resurfacing was carried out. The decision to close the carriageway was made after drivers reported hitting the jagged bump at speeds of up to 70mph.
A Highways Agency spokesman said: “It is still very early days and our investigators are looking at what caused this. Our priority was to get the road resurfaced so we could open it up again this morning.
“It is not something that happens very often. Generally when we have road surface problems it is due to a fuel spillage but this is a little more unexplained.
“We have completed the resurfacing to address the ridge in the road – two bits of tarmac had basically been pushed together and forced up.
“Heat is one of the things we’re looking into as a possible cause but it is not being addressed as the main factor at the moment.”