ZomNomNomI read that some motorcycles are about 90 decibels which is right under the average pain threshold for most people so I would probably suggest using the 15 or 21 decibel reduction ear plugs. This way it isn't harming your ears and you can still hear other traffic around you.
ZomNomNomi know this is probably a late response - i use a set of Earsonic EarPad plugs with Comply Isolation Foam T100 tips when I'm on my motorcycle.
the infographic on their amazon page claims an average of 16db reduction, however with the comply foam tips audio making it through is even quieter. I can still hear my motor some and other vehicles just fine. You could probably get the heavy metal ones and be just fine, protect that hearing.