The High Court in R (Transport Action Network Ltd) v Secretary of State for Transport [2021] EWHC 568 (Admin) considered the extent to which a party in judicial review proceedings may adduce expert evidence.

Key Points

  • Expert evidence is seldom required in order to resolve judicial review proceedings. That is because it is not the function of the Court in deciding such a claim to assess the merits of the decision of which judicial review is sought.
  • However where an understanding of technical matters is needed to enable the Court to understand the reasons relied on in making a decision in the context of a challenge to its rationality, expert evidence may be required to explain such technical matters.
  • Expert evidence may be needed if it is alleged that the decision under challenge was reached by a process of reasoning which involves a serious technical error.


The claimant, Transport Action Network Ltd (TAN), commenced judicial review proceedings against the Secretary of State for Transport regarding its roads investment strategy for the period 2020 to 2035 (“the RIS”). The grounds of challenge alleged a failure to take into account the impact of the RIS on achieving specific climate change objectives contrary to the Infrastructure Act 2015, which requires the Secretary of State to have regard to the effect of the RIS on the environment.

The Court granted permission for TAN’s claim to proceed to a hearing. TAN applied to rely on witness statements from two academics. It submitted that the further statements should be admitted in evidence in response to certain inaccuracies in the defendant’s evidence, and that they addressed the issues in the claim in that they set out the alleged errors of logic made by the Secretary of State in quantifying emissions arising from road schemes in the RIS and in assessing their impact on achieving the relevant environmental objectives.


The Court (Lang J) concluded that the two witness statements should be treated as expert evidence. The academics had no first-hand knowledge of the Secretary of State’s decision, but they possessed qualifications and expertise in the field and they were giving their opinions on the issues.

The Court agreed with the Secretary of State that in a “failure to have regard” challenge the focus should be on the material which the decision maker either did or did not take into account when making his or her decision. This was not an irrationality challenge in which the substance or the merits of the decision are reviewed by the Court. However in this case, the parties disagreed on whether the matters identified by TAN were mandatory material considerations. The defendant relied on the de minimis effect of RIS schemes on carbon emissions as a reason why these matters were not “obviously material” to RIS and, therefore, not mandatory material considerations. The Supreme Court in R (Friends of the Earth Ltd) v Heathrow Airport Ltd [2020] UKSC 52 confirmed that the test to decide whether a consideration is so “obviously material” that it must be taken into account is the irrationality test. Therefore, irrationality was in issue, albeit to a limited extent.

The Court also considered the decision of the Divisional Court in R (The Law Society) v The Lord Chancellor [2018] EWHC 2094 (Admin) where it was found that expert evidence may be needed if it is alleged that the decision under challenge was reached by a process of reasoning which involved a serious technical error. In this case, the Court concluded that the evidence from TAN's experts was on how and why the Secretary of State had allegedly seriously underestimated the emissions that would be caused by the RIS. The Court concluded that this was a technical field which a layperson, including the judge, could not fully understand without the benefit of expert evidence.


Given the nature of judicial review claims, expert evidence is rarely required in order to assist the Court to resolve the issues. This is because it is not the Court’s role to determine the merits of the decision under review, but rather it is concerned with the lawfulness of the exercise of the relevant public function. To answer that question, it is seldom necessary or appropriate for the Court to consider any evidence which goes beyond the material which was before the decision-maker and evidence of the process by which the decision was taken – let alone any expert evidence. However, as this judgment demonstrates, in limited circumstances the Court may require the assistance of an expert in order to make a determination.