The man on the Clapham omnibus
The man on the Clapham omnibus is a hypothetical reasonable person, used by the courts in English law where it is necessary to decide whether a party has acted as a reasonable person would — for example, in a civil action for negligence. The man on the Clapham omnibus is a reasonably educated and intelligent but nondescript person, against whom the defendant's conduct can be measured.
The term was introduced into English law during the Victorian era, and is still an important concept in British law. It is also used in other Commonwealth common law jurisdictions, sometimes with suitable modifications to the phrase as an aid local comprehension.
The phrase was first put to legal use in a reported judgement by Sir Richard Henn Collins MR in the 1903 English Court of Appeal libel case, McQuire v. Western Morning News. He attributed it to Lord Bowen, said to have coined it as junior counsel defending the Tichborne Claimant case in 1871. Brewer's also lists this as a possible first use.
It may be derived from the phrase "Public opinion ... is the opinion of the bald-headed man at the back of the omnibus", a description by the 19th-century journalist Walter Bagehot of a normal London man. Clapham, in South London, was at the time a nondescript commuter suburb seen to represent "ordinary" London. Omnibus is now a rather archaic term for a public bus, but was in common use by the judiciary at the beginning of the 20th century.
The use of the phrase was reviewed by the UK Supreme Court on appeal in the of case of Healthcare at Home Limited v. The Common Services Agency  UKSC 49, in paragraphs 1 to 4:
1. The Clapham omnibus has many passengers. The most venerable is the reasonable man, who was born during the reign of Victoria but remains in vigorous health. Amongst the other passengers are the right-thinking member of society, familiar from the law of defamation, the officious bystander, the reasonable parent, the reasonable landlord, and the fair-minded and informed observer, all of whom have had season tickets for many years.
2. The horse-drawn bus between Knightsbridge and Clapham, which Lord Bowen is thought to have had in mind, was real enough. But its most famous passenger, and the others I have mentioned, are legal fictions. They belong to an intellectual tradition of defining a legal standard by reference to a hypothetical person, which stretches back to the creation by Roman jurists of the figure of the bonus paterfamilias...
3. It follows from the nature of the reasonable man, as a means of describing a standard applied by the court, that it would be misconceived for a party to seek to lead evidence from actual passengers on the Clapham omnibus as to how they would have acted in a given situation or what they would have foreseen, in order to establish how the reasonable man would have acted or what he would have foreseen. Even if the party offered to prove that his witnesses were reasonable men, the evidence would be beside the point. The behaviour of the reasonable man is not established by the evidence of witnesses, but by the application of a legal standard by the court. The court may require to be informed by evidence of circumstances which bear on its application of the standard of the reasonable man in any particular case; but it is then for the court to determine the outcome, in those circumstances, of applying that impersonal standard.
4. In recent times, some additional passengers from the European Union have boarded the Clapham omnibus. This appeal is concerned with one of them: the reasonably well-informed and normally diligent tenderer.
In Australia, the "Clapham omnibus" expression has inspired the New South Wales and Victorian equivalents, "the man on the Bondi tram" (a now disused tram route in Sydney, New South Wales),  and "the man on the Bourke Street tram" (Melbourne, Victoria).
- Asprey, Michèle M. (2010) , Plain Language for Lawyers, Federation Press, p. 119, ISBN 978-1-86287-775-7
- Bagehot, Walter (1873) , The English Constitution, Little, Brown, and co, pp. 325–326
- Cameron, Donald M.; Renault, Ogilvy (30 December 2006), case comment: Beloit Canada Ltd. v. Valmet Oy citation(s): (1986) 8 C.P.R. (3d) 289, JurisDictioncom
- Handley, Justice J. (10 April 2013), Re Sortirios Pandos and Commonwealth of Australia  Australian Administrative Appeals Tribunal (AATA) 18 (25 January 1991), AustLII
- Room, Richard, ed. (1996), Brewer's Dictionary of Phrase and Fable (15th ed.), Cassell, p. 761, ISBN 0062701339
- SFAT (2009), Ng Chiu Mui v Securities and Futures Commission Application No 7 of 2007 (15 May 2009) (PDF), Securities and Futures Appeals Tribunal, p. 30
- UK Supreme Court (30 July 2014), Healthcare at Home Limited v. The Common Services Agency  UKSC 49 (PDF), pp. 2–3 § 1–4