Yes, there's even one in the area I was born (Pastuchow).
I'm not sure about the history though. I think when someone committed a crime, his/her penalty was to do a cross in the stone. I think it was self-penalty (similar to self-whipping in the times of the Middle Ages).
In my culture, Those crosses on the side of the road show to people who drive by that someone has been killed or either murder at such side. One in a while family and friends of the victim(s) go and arrange the crosses with flowers and pray. Some time also light up candles as they play.
Sincerely,
Anshka
Oksawa2, if you know some Polish, this will be a real treat:
krzyze.pl/index.php
if not, just e-mail the webmaster in English