Why is transporting hazardous materials that require placarding generally prohibited on a passenger-carrying vehicle?

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1. Placarded cargo can always be safely carried alongside passengers with no restriction, since placards are only required for interstate shipments
2. This restriction applies only to hazardous materials weighing over 500 pounds
3. Combining passengers with placarded hazardous cargo creates unacceptable risk if an incident with that cargo occurs
4. Passenger vehicles are exempt from all hazardous materials regulations

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