We must be thinking of two different things, because the
cattle grates I'm familiar with wouldn't exclude cars ... in fact, the whole point of them is to allow farm vehicles to pass without having to open and close a gate.
But I couldn't imagine even trying to ride over one on two wheels. At any speed. Even if you were uninjured, they'd taco your wheels.
ETA: but I guess it can be done.
http://jpschroeder.blogspot.com/2010...d-city-sd.html [scroll down to the last picture]