What is the test for reasonableness in a public entity's claim that it would have been too costly and impractical to do anything else?

To prevail in this defense, the public entity has the burden of showing it would have been " 'too costly and impractical . . . to have done anything else,' " i.e., that " 'under all the circumstances, including the alternative courses of action available to it and the practicability and cost of pursuing such alternatives, its action in creating or failing to remedy the condition was not unreasonable.' " (Metcalf v. County of San Joaquin (2008) 42 Cal.4th 1121, 1138.) The determination of reasonableness involves a weighing of "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." ( 835.4, subd. (b).)