I was a litle older, but I had in the back of my mind "someone might die in their sleep" so I would make sure to say a kind "good night" and "I love you" to my parents and siblings before heading off to bed. I have a feeling that type of anxiety, to a point, is normal.
However, whenever a "normal" anxiety or ritual or habit starts to interfere with normal day to day routines, then it is time to "worry". I would speak to the pediatrician (a phone call should be enough initially). He/She might be able to reasure you, or be able to refer you to a good councelor/psychologist. Seeking a professional's help may help you and your child figure out the root cause for her anxiety (maybe a house in the school bus goes past burned down). The professional can also help her find alternative ways to more appropriately feel safe without it becoming an obsessive ritual.
It might just be a phase that fizzles out in a few months, or it might not be. I'd call the pediatrician soon and get some advice. If you agree with the advice then great, and if you don't, then seek out another professional. I can only imagine getting help would only be good for everyone.
State Certified Early Childhood Special Education Teacher
New Mom as of March 2009!