Back in the old days, parking brakes could be used in an emergency. The hand brake on my Model A could lock up both rear tires. It used a separate circular shoe covering about 330 degrees, and had a toggle type actuator that spread the shoe against the drum.
The parking brake on my current truck won't even keep it from rolling back out of the driveway, so would be pretty much useless as far as slowing the truck down. I would hesitate to even refer to it as an emergency brake.
