TORTS – negligence – public and other authorities – whether the appellant was immune from statutory liability in the exercise of a special statutory power
26 First, as was touched on in Curtis, there is no bright line distinction between exercising a power and failing to exercise a power.
35 The test to be applied is, as noted in Curtis,  grammatically awkward. There are, three separate elements to the test which need to be identified. First, the requisite assessment of unreasonableness is to be made by an hypothetical reasonable public authority: while the court must make the assessment, it must do so by reference to the approach properly taken by such an authority. The significance of that element is that the exercise must be undertaken having regard to the limits beyond which a person (such as an authority) having necessary expertise in traffic engineering would not step.
36 Secondly, the test is formulated in the negative. By contrast with s 5B(1)(c) of the Civil Liability Act, it is not satisfied by evidence of what a reasonable traffic engineer would have done as a precaution against an identified risk. Rather, it is only satisfied by proof that no traffic engineer acting reasonably would have failed to take the precaution identified by the plaintiff. That is, accepting that there will be a range of views amongst reasonable traffic engineers, the omission must be such that no person with the requisite expertise could properly consider the omission to be reasonable. The fact that a high threshold is being prescribed is revealed by the double negative, “so unreasonable that no authority …”.
37 Thirdly, the section reformulates the standard by which a breach of duty is to be judged. Once the section is engaged, the plaintiff will have to establish negligence beyond the statutory threshold in order to succeed.  An assessment of the relevant evidence in the present case demonstrates that the plaintiff failed on any view to establish that burden. (Indeed, it may be doubted whether the evidence established breach according to the ordinary standard identified in s 5B.)