Kimmyt,
I think you should do it. Halloween costumes are supposed to be SCARY and getting hit by a car qualifies as very scary!
Besides, just when you think you've crossed the line to bad taste, that's when you find out most people really have a sick sense of humor.
My BF wore a costume to my own halloween party a couple years ago that was in such bad taste, I was embarassed to have people see him. It turned out his costume was the hit of the party!