Knitting with straight needles in a car is akin to running down the stairs with scissors. It takes very little to puncture the skin and injure you critically, and as someone already suggested, it could puncture the airbag. The forces in an accident are fara too great for you to control and the speeds with which things happen far too fast to respond too.
Remember too that hwile your driver may be an excellent driver, the roads are loaded with others who are not.