I don't think the helmet would have supported the weight on its own, but combined with the skull inside it's quite possible the helmet and its contents remained intact. Motorcycle helmets are real protection, unlike cycle helmets which are designed for 14mph knocks and nothing more serious. There could have been severe neck injuries though.
This is the exact stupidity I see on the roads every day, and why I get so angry at the recklessness. All from people bleating "we're allowed" and taking no responsibility for their own safety. And no, they AREN'T "allowed" to ride dangerously, but unless there's a specific law forbidding a particular action they're too thick to understand they shouldn't be doing it.
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