GOV §835.4: (a) A public entity is not liable under subdivision (a) of Section 835 for injury caused by a condition of its property if the public entity establishes that the act or omission that created the condi

(a) A public entity is not liable under subdivision (a) of Section 835 for injury caused by a condition of its property if the public entity establishes that the act or omission that created the condition was reasonable. The reasonableness of the act or omission that created the condition shall be determined by weighing the probability and gravity of potential injury to persons and property foreseeably exposed to the risk of injury against the practicability and cost of taking alternative action that would not create the risk of injury or of protecting against the risk of injury. (b) A public entity is not liable under subdivision (b) of Section 835 for injury caused by a dangerous condition of its property if the public entity establishes that the action it took to protect against the risk of injury created by the condition or its failure to take such action was reasonable. The reasonableness of the action or inaction of the public entity shall be determined by taking into consideration the time and opportunity it had to take action and by weighing the probability and gravity of potential injury to persons and property foreseeably exposed to the risk of injury against the practicability and cost of protecting against the risk of such injury.