Banners are being put up around two Gloucestershire commons this weekend warning motorists more than 300 head of cattle are being put out to graze.
The banners are being put up by firefighters at the weekend
The cows are due to be released onto Minchinhampton and Rodborough commons on Sunday morning.
Firefighters are placing banners at the entrances to both commons. Station Commander Andy Gillingham hoped motorists would heed the 40mph limit.
"They are big animals which if hit, can cause serious injuries," he said.
"The brigade deals with the aftermath of accidents and we hope this publicity will make people think about driving safely across both Rodborough and Minchinhampton commons," Mr Gillingham added.
To alert motorists the signs bearing the 40mph limit a slogan "We are out now - moooove safely!" are being put up on the affected roads.
Stan Waddington from Gloucestershire County Council, said: "The grazing of cattle is essential to preserve the unique ecology of the commons and, of course, represents someone's livelihood.
"A significant number of cattle are going to be released and I would urge people to respect the cattle and to be especially careful.
"Drivers must give themselves plenty of time to react to the sudden appearance of animals on the road. To not do so is to put themselves and the livestock at risk," he said.