Interim Garda Commissioner Noirín O'Sullivan has told the Public Accounts Committee that the reason that receipts from speeding tickets from the outsourced speeding camera system GoSafe are down is because drivers are observing the speed limits in those areas.
The cameras are in 518 zones where serious accidents have occurred.
Fine Gael Deputy Áine Collins asked why GoSafe vans were detecting one in 40 drivers while the figures for the garda speed vans was much higher.
Acting Commissioner O'Sullivan explained this was because people knew where the GoSafe cameras are, but they do not know where the garda speeding vans are.
She added that the GoSafe cameras are achieving the objective of compliance behaviour, reduction of speed and as a consequence less lives lost.
Going through the annual accounts, the Comptroller and Auditor General Seamus McCarthy told the committee that it was first envisaged that receipts from the GoSafe system would be more than enough to cover the cost.
However, as speed detection dropped there was a shortfall of 30% in revenue collected to pay for the system and this had to come out of receipts from other fixed penalty notices that would have otherwise gone to the Exchequer.
He added that a review outlining the value of the GoSafe contract would be timely as it is coming towards the end of the five-year contract.
Ms O'Sullivan told Deputy Collins that speed compliance rates in the zones ranged from 81-90% and it was not projected that compliance rates would be so high. As a result, revenue dropped for the project.