You subject your spine and head to more impact forces walking down stairs than you can experience in a car starting or stopping quickly. The most a car can achieve is 1.7 Gs. You can experience 3 Gs or much more just walking down stairs. The difference is you are prepared for walking down steps. In a car, you are rarely prepared. The driver is but the passengers are not. So, the passenger is startled by the movement. The startle causes muscle tightening and sometimes, an anxiety event.