Worst evil booster with lap belt or lap belt with no booster

[joolsplus3]

Tangential thing ive wondered: if a booster were unavailable, which is preferable, lap belt or full seatbelt? My instinct is to say full seatbelt, but i wanted to check. I was riding with a friend once who let her 4-5yo ride in a lap belt ssying, "im okay with no booster sometimes, as long as its a lap belt"and i was thinking, she was way off. Ive heard other people justify skipping a booster just because its a lap belt. Im thinking the hierarchy would be booster w/ full seat belt, as best, full seat belt w/o booster in a pinch/emergency, lap belt w/ no booster would be absolute last resort...do i have this right?

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The full seatbelt is still preferable, she's wrong. The shoulderbelt will both protect the head and upper body and spread the forces out from the lapbelt. Maybe send her the link above about seatbelt syndrome to help educate her? Even my mom (who's learned a ton from me!) thought the shoulderbelt would decapitate a smaller kid, and I think that's the misinformation that is still out there.