In my experience, there are two issues.

Posture is important for relaxing and sleep. A cervical collar/neck brace may help during sleep and relaxing if it fits and support correctly. A travelers sleep pillow can help when flying.

Impact protection in a car is different. A collar can make a jarring more intense because there is less ability to flex and reduce the forces. I find it is better to ride in the car with shoulders off the seat back, slightly hunched forward. This allows the spine to flex and absorb a large amount of jarring energy.

Some have used seat cushions to reduce vertical forces. It needs to be stiff enough to still have some absorbing ability after being compressed by sitting on it. Many cushions are just too soft.