The bus was on the A4, one of the major highways leading from the Hungarian border to Zagreb, near the village of Breznica which looks to be about halfway between Zagreb and Varazdin. It was at 5:25 a.m. local time. I don’t know if it would have been light by then—Breznica is about 46 degrees North latitude and it’s toward the eastern end of that time zone so it may get light pretty early.
Just checked a couple of the Croatian websites for more news about the bus crash. Apparently they haven’t determined the cause of the crash yet. The driver was over 70 years old. A couple of the injured are in very serious condition, others are stable and some have returned to Poland.