More than half of Go Safe speed detecting vans could be unavailable due to industrial action from Saturday.
SIPTU confirmed that a 72-hour work stoppage by its members will go ahead from 9am on Saturday, October 26.
The Services, Industrial, Professional and Technical Union is in a row over working conditions and union recognition.
SIPTU's Brendan Carr said the issue has been ongoing for a number of years.
"We've been over two years trying to resolve the problems."
He said that management are "refusing to engage to try and resolve some very very serious health and safety issues".