Despite popular belief, a person or a business, as the case may be, can not exclude or limit its liability for moral or bodily injury to others. This means that despite the signing of such a “discharge”, you still have recourse against the author of the damage in the event that an injury occurs through his fault. Indeed, one can not, in advance, exclude his responsibility. Of course, you should not have contributed to this injury through your fault or negligence.
On the other hand, as you were aware of the risks inherent in such activity and have knowingly agreed to it, this may be a mitigation of the liability of the wrongdoer, without, however, waiving your right of recourse.
However, a distinction must be made when it comes to material damages, for example the sign indicating “we are not responsible for lost or stolen objects” since in these cases and provided that the poster is visible, it is indeed possible to limit your liability and not to be held responsible for the artwork stolen from your trunk, for example.