The changes, which began to be introduced in September, follows the fitting of new lighting systems, along with the replacement of existing lamp-posts and older-style bulbs.
Cost was not was the overriding reason, said the State agency, though the changes were spurred by the need to comply with a 2012 EU energy policy that requires Ireland to cut greenhouse gas emissions.
Ireland has 180 motorway junctions. Those created before 2007 required motorways to be lit three kilometres before junctions, though those built afterwards were to have lights only at ramps, and connecting junctions.
However, just a quarter of those built after 2007 were actually lit to the later standard. Instead, nearly half had bespoke lighting systems installed. Thirty per cent were built to the pre-2007 standards.
Lighting each pre-2007 junction used 150,000 kWh of electricity each year, costing €25,000. Those fitted to 2007 standards cost €16,000 per year. Extra savings in servicing are also expected.
Four junctions were converted in a pilot programme: Junction 6 Balbriggan on the M1; Junction 9 Enfield on the M4; Junction 3 Rochfortbridge on the M6 and Junction 3 Athy on the M9. It has now been extended to 24 junctions.
Following a year-long test no negative results were found. Indeed, the new lighting is safer, said the State agency, because motorists face less glare and fewer lamp-posts are needed.
In addition to the elimination of surplus lighting, off-peak dimming at suitable locations and the use of LED light fittings is being explored. A pilot dimming project began on the M50 in 2017 and incorporates use of voltage regulation and dimming appropriate to traffic flows.