Failure to introduce measures to reduce car usage beyond current plans to improve public transport and cycling infrastructure will cause transport emissions to grow 15pc by 2030, forecasts show.
That contrasts with the 50pc reduction the sector is supposed to achieve by the end of the decade.
The bleak scenario is set out in the same National Transport Authority (NTA) modelling that has caused controversy for looking to radical measures to deter people from driving.
Environment and Transport Minister Eamon Ryan will ask the Cabinet today to approve the drawing-up of a strategy to persuade people away from using cars where possible.
His request will cite the NTA’s modelling and the potential impact that measures such as reduced parking spaces, increased parking charges, no-car zones, congestion charges and other deterrents could have on emissions.
Reaction has been swift with the AA, Fine Gael politicians and Independents arguing that motorists should not be forced to pay more, particularly where public transport options are poor.
But the NTA modelling shows a price to be paid for shying away from tougher measures.
It says that even with the BusConnects projects and other planned public and active transport improvements, total kilometres driven by car will increase by 3pc by 2025 and by 12pc by 2030.
Kilometres covered by goods vehicles will increase by 16pc and 26pc in the same time periods.
Those calculations are based on population growth of 6pc by 2025 and 10pc by 2030, and employment growth of 11pc and 18pc over the same periods.
That would result in 1.84 million tonnes (mt) more carbon emissions, on top of the 12.2mt currently produced by transport, a 15pc increase.
The NTA report shows that a 50pc reduction is possible through a combination of measures.
“Fleet improvements”, which means more electric vehicles and improvements in the efficiency of petrol and diesel engines, could save 4.75mt.
Greater use of biofuels in petrol and diesel vehicles, already part of the plans, could save another 1.08mt.
Reducing speed limits, which enables more efficient use of petrol and diesel, could take out another 0.14mt, while “behavioural change measures” could save a further 1.97mt.
“Behavioural change” would be achieved by measures such as greater pedestrianisation of urban centres, congestion charging and parking constraints, but also by incentives such as reduced public transport fares and increased working from home.
While government policy has placed much emphasis on switching to electric vehicles, the report says that even presuming 100pc of new car sales were electric by 2029 (80pc fully electric and 20pc plug-in hybrids), they would still only account for 30pc of all cars on the road.
It also says that greater impact in terms of emissions reduction might be achieved if incentives to buy EVs were targeted at those driving the furthest distances.
It measures the average weekday distance travelled by car in rural areas at 43.8km compared to 17km in Dublin city and 18.1km in other cities.
“Promoting a shift to electric vehicles in particular areas can potentially be a more effective policy than a blanket incentive and could therefore maximise the impact of the fleet electrification,” it said.