One word: caltrops.
Caltrops.....
Never heard the proper name before. Thought everyone called them Jack Rocks. A bit of Google yielded this:
Way back when (sometime during the late 1980’s), Virginia coalminers decided to strike, becoming rowdy in the process, and when the state police moved in to restore order they were met by jack rocks in the roadways (jack rocks are large, sharpened metal objects shaped like jacks—kid toys—designed to flatten car tires), gunfire, and incoming spuds fired from potato cannons.
The VSP spent nearly $200,000 to replace flattened car tires during the period when over 400 troopers were assigned to the area on a rotating basis.