Regardless of what type of carrier you use, all formats should be placed in a secure space, when travelling in the car. In the case of these backpacks, they should always be placed on a flat surface, where you can be sure they won't slide - even if you hit the brakes.
It is not recommended that these backpacks are used as a car seat, as standalone products. However, if you are able to strap these down securely and are confident in the safety of your cat when driving, you can use seatbelts to secure the backpack and ensure the safety of your cat.
Generally, the best way to do this is to loop the seatbelt through the straps of the carrier - both the short and long sections of these - in order to secure your backpacks. If you can't do this, keeping your cat in the footwell of the back seat is a good alternative, which will ensure you're not distracted while driving.