The use of mobile speed cameras in Somerset has risen by 10% in two years.
Figures obtained by the BBC show the cameras were used at 50 sites across the county and detected nearly 17,000 speeding offences between 2007-09.
It also emerged that the use of fixed cameras - of which a third are being removed because they have "done their job" - stayed the same.
Road Safety GB said the flexibility of mobile cameras made them more attractive to operators.
The Safety Camera Partnership - which runs the speed cameras - was unavailable for comment.
Nine out of 27 fixed cameras are being taken down because operators say they have reduced the number of crashes and casualties in the county.