Time-Saving Bias: Why Speeding Up Saves Less Time Than It Feels

The arithmetic is the whole story, and it is counterintuitive. The time a trip takes is distance divided by speed, so the time saved by going from one speed to another is the distance multiplied by the difference between the two inverse speeds. Over 100 miles, going from 40 to 50 mph saves 30 minutes; going from 80 to 90 mph, the same 10 mph gain, saves 8 minutes and 20 seconds. The same increase buys less and less as the starting speed rises, because each extra mile per hour is a smaller share of a larger number.

Intuition does not work with inverses. Svenson showed in 1970 that people's estimates of time saved fit a proportion heuristic: the saving is judged as the proportion the speed increase represents, with the starting speed given too little weight. In his 2008 paper, which named the bias, he showed that in a choice between two speed increases this heuristic simplifies to a ratio rule, and that the rule predicted the wrong choices people made. Peer tested the proportion heuristic against a simpler difference heuristic, in which only the size of the increase matters, and found some evidence for the latter in 2010; either way, the starting speed is neglected, and neglecting it produces the two halves of the bias at once. From a low starting speed the saving is underestimated; from a high one it is overestimated.

The pattern is not confined to drivers answering questionnaires. Fuller and colleagues put four speed-change questions to a representative sample of 1,005 UK drivers in 2009 and found that drivers believed a speed rise saved more time the higher the starting speed, the opposite of the truth. Peer and Solomon interviewed taxi drivers mid-journey in 2012 and found the same misestimations of speed, journey time and time saved as among ordinary drivers, with somewhat smaller overestimates. Eriksson, Svenson and Eriksson put the task into a driving simulator in 2013, asking drivers to repeat a stretch of road at whatever speed would gain exactly three minutes, and the bias survived the experience of actually driving.

The same mistake appears wherever a rate is quoted. Svenson showed it in planning doctors' time in clinics and in choosing which production line to speed up, and de Langhe and Puntoni found in 2016 that consumers given productivity metrics such as megabits per second paid more for increases at the high end of the range, where the time saved is smallest.

Ola Svenson published "Decisions among time saving options: When intuition is strong and wrong" in Acta Psychologica in 2008. The paper built on his 1970 finding, from a functional measurement study in the Journal of Experimental Psychology, that estimates of time saved by increasing driving speed follow a proportion heuristic rather than the correct inverse relation. The 2008 paper asked what that heuristic does when people are not estimating a saving but choosing between two of them.

Three studies answered. In the first, participants chose between pairs of road improvements described by the mean speeds they would produce, and the ratio rule derived from the proportion heuristic predicted their choices, which were the wrong ones: the option that raised the already higher speed by a larger proportion was preferred although the option that raised the lower speed saved more time. The second study moved the problem to health care, asking which of two clinics should be reorganized to free more of the doctors' time for patients, and found the same bias. The third used a matching procedure in which participants adjusted one option until it seemed to save as much time as the other, and the matches again followed the ratio rule. Svenson concluded that the studies had illustrated a time-saving bias and explained why people make systematic errors when judging and deciding about time saved after a speed increase.

The figure on the result screen comes from Peer's 2010 study of 79 drivers in Judgment and Decision Making, whose appendix reports every question and mean response; 75 of the 79 answered the item quoted.

The rows below are illustrations of decisions in which a rate stands in for a time. They are not findings from the studies cited on this page, except where one is named.

The bias comes from the unit, so the remedy is to change the unit.

• Think in pace, not speed. Minutes per mile, or minutes per 10 miles, is proportional to time, so differences in pace are differences in time. Peer and Gamliel showed in 2013 that drivers given pace information estimated journey times, time savings and required speeds more accurately.
• When a choice is between two speed increases, compute both trip times. The saving is the difference between distance divided by the old speed and distance divided by the new speed.
• Remember the direction of the error: a gain from a low speed is worth more than it feels, and a gain from a high speed is worth less. If you are already fast, the next increment is the least valuable one.
• When you buy anything sold by rate, megabits per second, pages per minute, items per hour, ask for the time a typical job takes. The time is what you are paying for.
• When speed costs risk, as it does on the road, set the saving against the cost in the same units. Svenson's 2009 participants underestimated the growth in stopping distance and in crash risk with speed, and overestimated the time saved.

The time-saving bias is a specific error about rates and inverses. Its neighbors concern time and quantities in other ways.

• It is not the planning fallacy, the underestimation of how long a task will take. Svenson connected the two, since someone who overestimates what speeding up will recover may plan too little time, but the planning fallacy appears with no speed change involved.
• It is not the Weber-Fechner law, the finding that a difference is judged against its base, so that 10 mph seems smaller at 80 than at 40. That judgment of the increase is accurate as far as it goes; the time-saving bias is the failure to invert it, since the smaller-seeming increase at 80 also saves less time.
• It is not money illusion, where a nominal figure is mistaken for its real value. The two share a shape, a visible number standing in for the quantity that matters, but money illusion is about prices and the time-saving bias is about rates.