The government has also doubled the hours of operation by implementing a double-shift and have added almost 300 new sites to the program, an ACT Legislative Assembly committee heard this week.

"A couple of things came out of the road safety camera program, particularly that the mobile speed vans are the most effective in deterring speeding," ACT road safety minister Shane Rattenbury said.

"The data shows when the vans are more visible you do see a downturn in speeding behaviour and so that is why the government has focused on increasing mobile speed vans and delivering a message that they can be anywhere, anytime."

Acting director-general for the Justice and Community Safety Directorate David Pryce said there had been a 25 per cent to 35 per cent reduction in serious injury crashes on roads where the cameras were being used, which was "very significant".

"To date 292 new sites have been added to the program since regulation changes commenced in October 2015, supporting the use of mobile speed cameras units on any road in the ACT and 65 of those new sites are at school zones," Mr Pryce said.

However deputy executive director Karen Greenhill said not all schools were able to have mobile speed cameras outside.

"My understanding is there are one or two schools where it isn't possible to find a suitable site, one of the factors that's relevant is that the van has to be placed somewhere it doesn't create an issue, line of sight issues and safety for people in the area as well as some OHS issues for people operating the camera in the van," Ms Greenhill said.