Not sure what happened here. I had my 12 seater backed into my driveway with the parking brake engaged and the van in park. There was a little snow on the driveway as it had been snowing, but everything was before shabbos. Late shabbos afternoon around mincha time I saw that the van was sliding into the street!! Baruch Hashem it stopped at the end of the driveway where the ground was level but it was terrifying to think that it could have rolled straight across the street!