After seeing this question about Yugoslavian monuments, I'd like to go check out the monument at Petrova Gora:
It seems there are regular buses and trains to Karlovac, but that's still an hour's drive away. Is there any way to get closer to Petrova Gora by public transport?
The closest is the bus line Karlovac-Topusko. This passes Vojnić (where the only access road to the monument starts) and continues along the road to the north of the monument. A stop like Biljeg looks like a good starting point for a hike. To the best of my knowledge there is no bus on the road leading up to the monument. You could try to hitch a ride up there from Vojnić or take a taxi if you want to avoid the hike.
You can find the bus schedule on BalkanViator. Every day there are many buses on that line.
