The council said the appearance of the signs was "a bit of a mystery"
Signs put up at a set of roadworks in Cambridgeshire warning drivers of a new speed limit were in Welsh.
Four signs in Welsh were placed on the Longthorpe Parkway in Peterborough with the English translation underneath.
A number of drivers who saw the signs contacted the city council and some took pictures.
A council spokeswoman said: "They seem to have just appeared there. We're not too sure how and why. It's all a bit of a mystery."
She said the roadworks were carried out by a contractor and the road had been closed overnight for the work to take place.
"The road was closed and when it re-opened there were no speed restrictions in force.
"So therefore there was no need for signs. Why speed restriction signs would be put up we have no idea."
A council spokesman added while it could have been the contractors hired to carry out the work it also could have been a practical joke.
The signs have now been removed.