As my grates are stolen I have replaced them with fiberglass. They haven't taken the fiberglass ones yet.
The ones that I've replaced were 3'x3' and 2'x4'. I have driven right over the center of them many times without any problems. Oldest ones are probably 6-7 years old.
Any chance that you have a link to those specific fiberglass grates that can be driven over by possibly a much heavier vehicle than the average car etc.?