My knowledge of Bulgaria is a little dated now, though I aim to update this page later this year. I cycled from Sofia to Nis in Serbia on the A6 which was flat and featureless in Bulgaria, and very dangerous on the Serbian side as it weaved through the canyon and tunnels, very busy with HGVs and tourist buses.
That road has since been replaced by the “European Motorway”. It’s still possible to take that route (the border is open to cyclists) using the old A6 where it still exists. Where it doesn’t, there’s a cycle path alongside, in the Polish brick-paved style. Slightly longer routes away from the motorway would be more pleasant.
