They use big rocks here. Things that weigh a significant part of the weight of
a car but being low they duplicate the things used in NHTSC tests with less
than half the kinetics, so the car does OK.
A post can slice through a car.

Yup! Rocks, not big enough to kill anyone, just big enough to screw up their
car when they drive over them. Basket ball size. Add some landscaping, a few
small bushes and some flowers. After they get towed off your rocks you can
get their insurance information for the damage caused to your landscape.
Those proffesional landscape companies really charge some big bucks!
You may even make a few dollars in the end!
Greg