Health promotion groups have called for the "urgent implementation" of BusConnects, the Government's plan to improve public transport and reduce carbon emissions.
The Active Travel Coalition - established in 2021 - said it is concerned that political support has waned with projects "at serious risk of being delayed or watered down unnecessarily".
The alliance includes the Irish Heart Foundation, the Irish College of General Practitioners, Irish Doctors for the Environment and Cyclist,ie, the national cycling advocacy network.
The coalition said that BusConnects - if fully implemented - would radically improve public transport, increase walking and cycling, and reduce carbon emissions in Ireland's major urban centres.
Mairéad Forsythe, of Cyclist.ie, said: "There's a lot of political opposition to some of the plans and planning applications may be rejected.
"Our fear is that if that happens, projects will never go ahead and we'll be left with the poor public transport system we have which will push people to use their cars".
BusConnects aims to reorganise routes in five cities as well as providing more bus corridors, cycles lanes, and improved footpaths and crossings for pedestrians.
The National Transport Authority (NTA) said that five of a planned 11 phases of the project have been implemented, including 24-hour bus services in Dublin.
It said that construction of bus corridors cannot start until approved by An Bord Pleanála.
Some of the rollout has been impacted by a shortage of bus drivers, the NTA added.
BusConnects aims to deliver 230km of bus corridors and 200km of cycling lanes, resulting in an increase of almost 25% in bus services.
Last year, the Government approved measures to allow for planning applications for bus corridors under the programme to proceed.
The move also gave the NTA approval to begin the procurement for "next generation ticketing" using mobile phones.
Due to the size of the programme, construction of the bus corridors has been phased into three groups to reduce disruption to commuters.
