I think without a rollcage a harness is more dangerous in the event of a flip. Harness would prevent you from making any attempt to move from the impact - either reflex action or intentional. You'd have zero alternative but to take the full impact on the top of your head.

And then there's the question of how the harness is fixed. If its a dodgy clip into rear parcel shelf, then its probably not going to restrain you at any imapct at speeds close to speed limit. Guess that equation all depends on weight of driver and other factors...

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meh