The National Transport Authority said more lanes of car traffic may be lost in Dublin to give way to buses.
New figures show some journeys along Quality Bus Corridors took longer last year than in 2015.
This is because many double-deckers have to share the road with cars for large parts of the route.
CEO Anne Graham said the Authority will use money ring-fenced in the budget to start making changes next year:
"It could be that a lane of car traffic may have to be given up for a bus or cycling. Looking at car lanes, looking at where there might be a loss of parking, some road widening, signal junctions, other things might be changing. All of that will be revealed in early 2018," she said