The Paceometer Effect

The Paceometer Effect

Drivers badly misjudge how much time they will save by going faster, because a speedometer shows distance per unit of time while the quantity a driver is actually deciding about is time per unit of distance. Eyal Peer and Eyal Gamliel demonstrated it in 2013 and showed that changing the display to minutes per fixed distance improves the judgements sharply. Peer is first author. The arithmetic is not in dispute and the direction of the error is well established, but the demonstration was run on judgement tasks rather than on roads, so treat it as a finding about instruments and not as a finding about driving.

What it is

The error has a consistent direction. Raising speed from an already high speed saves far less time than people expect, and raising it from a low speed saves far more. That is why the last stretch of motorway acceleration buys so little and why crawling traffic is so costly. A speedometer displays the reciprocal of the thing the driver cares about, and people do not invert reciprocals well in their heads.

Peer and Gamliel tested a display showing minutes per ten miles rather than miles per hour, and the proportion of correct time-saving judgements rose from under a fifth in the control conditions to well over half. The work builds explicitly on the miles-per-gallon illusion, published five years earlier by Richard Larrick and Jack Soll, where fuel economy expressed as distance per volume produces the same structural mistake about fuel saved. Two research groups arriving at the same error from different units is a good sign for the underlying claim.

In effect

The generalisable rule is the reason to keep the concept at all. When people have to judge a saving, give them the quantity they are trying to save, in the units they are trying to save it in. Time saved is a time problem and belongs in minutes. Fuel saved is a volume problem and belongs in gallons or litres. Any rate that inverts the terms of the decision will produce this error, and the remedy is always a change to the instrument rather than a change to the person.

The vault also holds this concept as a sourcing case, and the case is about an apparatus rather than about anyone's competence. Hasard Lee's The Art of Clear Thinking gives the finding twice within two sentences, both times in the passive register of research, and names neither researcher. It is the one flagged claim in that book that is traceable through its bibliography, and the entry there points at a 2012 record in a document repository rather than at the journal article. The correct citation is Peer and Gamliel in Judgment and Decision Making, 2013, and the attribution has to be reverse-engineered from a chapter list because the names never appear in the text.

What it does not say

It does not say that a paceometer display has been fitted to vehicles and changed how anyone drives. No such trial was found. The improvement was measured on judgement tasks, and a gain on the task does not transfer to the road by default. Far transfer is exactly where claims of this shape usually fail, and nothing here establishes it.

It does not say that drivers are innumerate. The error is produced by the display, which is why the fix is a display and not a lecture.

It does not carry any recommendation about speed. The finding is about how a saving is judged, not about what saving is worth taking.

Sources

  1. Peer, E., & Gamliel, E. (2013). "Pace yourself: Improving time-saving judgments when increasing activity speed." Judgment and Decision Making, 8(2), 106-115. Peer is first author. A minutes-per-ten-miles display raised correct time-saving judgements to roughly 58 per cent, against under 20 per cent in the control conditions.
  2. Larrick, R. P., & Soll, J. B. (2008). The miles-per-gallon illusion, the parent finding this work builds on.
  3. Lee, H. The Art of Clear Thinking, p. 70. The finding is given twice as "researchers found", with neither researcher named. The chapter 2 bibliography entry carries the right title and a DOI but cites a 2012 PsycEXTRA record rather than the journal publication. Cite Peer and Gamliel.
  4. No evidence was found that a paceometer display has been fitted in vehicles or has changed driving behaviour.