Your employer would not be liable if the accident occured because you were tired and fell asleep or had slowed reactions due to long shifts. Employers have the right to make employees work overtime; genereally speaking, there is no liability for 1) doing something which you have a legal right to do; or 2) when another person chooses voluntarily to do the allegedly risky activity (that is, you could refuse to work overtime if you feel it's dangerous, even if that costs you your job--the law expects people to not put their jobs ahead of safety).