according to NHRA rule, 9.99 to 7.50 and 135 and over mph need certified every 3 years. window nets are required in any full bodied car that is REQUIRED by the rules to have a roll cage. so if you are running 11 sec or slower and you have a cage, you don't have to have a net. but then again, you can never be too safe. so to answer your ?, if you're running 11.49 to 11.00, you need a roll bar, faster than that, you need a cage. faster than 10, needs cert.