Can State be held liable for rock slides on a highway?

Its impossible to determine,  based on what you wrote, if the State has liability.  However, there is one thing you need to know.  The type of liability you are talking about is called a "tort".  When you claim a public entity, such as the State of New Jersey, has committed a tort, you have to file a special notice against the State called a Tort Claim notice.  This notice has to be filed within 90 days of the event occurring, no exceptions.  This is a link to a form. http://www.nj.gov/military/vmh-policies/Business%20Office%2044-02-004B.pdf So your first step in resolving the liability issue will be to file this notice and then the State's insurer may contact you regarding the road.  Good luck !