California Government Code Section 830.6, which codifies the defense of design immunity, provides in relevant part: "Neither a public entity nor a public employee is liable under this chapter for an injury caused by the plan or design of a construction of, or an improvement to, public property where such plan or design has been approved in advance of the construction or improvement by the legislative body of the public entity or by some other body or employee exercising discretionary authority to give such approval or where such plan or design is prepared in conformity with standards previously so approved, if the trial or appellate court determines that there is any substantial evidence upon the basis of which (a) a reasonable public employee could have adopted the plan or design or the standards therefor or (b) a reasonable legislative body or other body or employee could have approved the plan or design or the standards therefor." In Cornette v. Department of Transportation (2001) 26 Cal.4th 63, the Supreme Court explained the purpose of the design immunity defense as follows: "The rationale for design immunity is to prevent a jury from second-guessing the decision of a public entity by reviewing the identical questions of risk that had previously been considered by the government officers who adopted or approved the plan or design. ' " 'To permit reexamination in tort litigation of particular discretionary decisions where reasonable men may differ as to how the discretion should be exercised would create too great a danger of impolitic interference with the freedom of decision-making by those public officials in whom the function of making such decisions has been vested.' " ' " (Id. at p. 69.) "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. " (Cornette, supra, 26 Cal.4th at p. 69.) In Cameron v. State of California (1972) 7 Cal.3d 318, the plaintiffs suffered injuries in an automobile accident that occurred after the driver of the vehicle in which they were traveling lost control of the vehicle while negotiating a curve on a highway. (Cameron, supra, at p. 321.) The plaintiffs brought a cause of action against the state alleging that it "failed in its duty to keep the highway in a safe condition in that the curve was so improperly graded or banked that an automobile could not negotiate the curve even though going at a lawful speed." (Id. at p. 322.) The trial court concluded that the state established as a matter of law all of the elements of its design immunity defense. (Id. at p. 322, fn. 3.) On appeal, the Supreme Court agreed with the plaintiffs "that the design immunity conferred by Government Code section 830.6 is inapplicable since the design plan approved ... did not specify the degree of superelevation on the curve and since it was the improper superelevation which constituted the dangerous condition causing the accident." (Id. at p. 322.)