What is the burden of a public entity seeking design immunity?

"`In other words, a public entity claiming design immunity must establish three elements: (1) a causal relationship between the plan or design and the accident; (2) discretionary approval of the plan or design prior to construction; and (3) substantial evidence supporting the reasonableness of the plan or design.' [Citation.]" (Weinstein v. Department of Transportation (2006) 139 Cal.App.4th 52, 58.) Defendant bears the burden "of establishing each element of its design immunity defense in support of its summary judgment motion. . . ." (Id. at p. 60.)