The legislative basis for the use of speed cameras is set out in section 81 of the Road Traffic Act 2010. Subsection (7) of that section provides for the Minister for Justice to enter into agreement with operators of the cameras.
A number of requirements for the content of such an agreement are set out in the Act, but there is no requirement to make the speed vans visible to oncoming traffic and I have no plans to create such a requirement.
I would expect that most drivers as a matter of course drive within the legal speed limit, both because it is the law and because it is safer. Where people do not, the deterrent effect of seeing a speed van would be limited to the period when they knew they were within sight of the van. The deterrent effect is obviously stronger if speed vans are strategically located at locations determined by An Garda Síochána. This strategic approach lends added road safety benefits for all those using the road network by ensuring that those drivers who may be tempted to exceed a given speed limit are less inclined to do so.