Time-saving bias

Cognitive bias

Time-saving bias is a concept that describes people's tendency to misestimate the time that could be saved (or lost) when increasing (or decreasing) speed.[1][2]

In general, people underestimate the time that could be saved when increasing from a relatively low speed—e.g., 25 mph (40 km/h) or 40 mph (64 km/h)—and overestimate the time that could be saved when increasing from a relatively high speed—e.g., 55 mph (89 km/h) or 90 mph (140 km/h). People also underestimate the time that could be lost when decreasing from a low speed and overestimate the time that could be lost when decreasing from a high speed.

## Examples

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In one study, participants were asked to judge which of two road improvement plans would be more efficient in reducing mean journey time. Respondents preferred a plan that would increase the mean speed from 70 to 110 km/h (43 to 68 mph) more than a plan that would increase the mean speed from 30 to 40 km/h (19 to 25 mph), although the latter actually saves more time.[3]

In another study, drivers were asked to indicate how much time they felt could be saved when increasing from either a low (30 mph (48 km/h)) or high (60 mph (97 km/h)) speed.[4] For example, participants were asked the following question: "You are driving along an open road. How much time do you feel you would gain if you drove for 10 miles [16 km] at 40 mph [64 km/h] instead of 30 mph [48 km/h]?".[5] Another question had a higher starting speed of 60 mph (97 km/h), and two other questions asked about losing time when decreasing speed, from either 30 or 60 mph (48 or 97 km/h).

Results supported the predictions of the time-saving bias, as participants underestimated the time saved when increasing from a low speed and overestimated the time saved when increasing from a relatively high speed. In addition, participants also misestimated the time lost when decreasing speed: they generally underestimated the time lost when decreasing from a low speed and overestimated the time lost when decreasing from a relatively high speed.[4]

## Explanation

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Time savings from extra 10 mph (16 km/h) decrease as speed increases.

The physical formula for calculating the time,  t {\displaystyle t} , gained when increasing speed is:

t = c D ( v 1 − 1 − v 2 − 1 ) {\displaystyle t=cD(v_{1}^{-1}-v_{2}^{-1})}

Where  c {\displaystyle c} is constant and used to transform between units of measurement,  t {\displaystyle t} is the time gained,  D {\displaystyle D} is the distance traveled and  v 1 {\displaystyle v_{1}} and  v 2 {\displaystyle v_{2}} are the original and increased speeds, respectively. This formula shows the relationship between increasing speed and journey time is curvilinear: a similar speed increase would result in more time saved when increasing from a low speed compared to a higher speed. For example, when increasing 20 to 30 mph (32 to 48 km/h) the time required to complete 10 miles (16 km) decreases from 30 to 20 minutes, saving 10 minutes. However, the same speed increase of 10 mph (16 km/h) would result in less time saved if the initial speed is higher—e.g., only 2 minutes saved when increasing from 50 to 60 mph (80 to 97 km/h). Changing the distance of the journey from 10 miles (16 km) to a longer or shorter distance will increase or decrease these time savings, but will not affect the relationship between speed and time savings.

Svenson suggested that people's judgments of time-savings actually follow a Proportion heuristic, by which people judge the time saved as the proportion of the speed increase from the initial speed.[6] Another study suggested that people might follow a simpler difference heuristic, by which, they judge the time saved based solely on the difference between the initial and higher speed.[7]

It seems that people falsely believe that journey time decreases somewhat linearly as driving speed increases, irrespective of the initial speed, causing the time-saving bias. Although it is still unclear what is the dominant heuristic people use to estimate time savings, it is evident that almost none follow the above curvilinear relationship.

## Consequences in driving

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Drivers who underestimated the time saved when increasing from a low speed or overestimated the time lost when decreasing from a high speed, overestimated the speed required for arriving on a specific time and chose unduly high speeds, sometimes even exceeding the stated speed limit.[2] Similarly, drivers who overestimated the time saved when increasing from a high speed underestimated the speed required for arriving on time and chose lower speeds.[8]

## Consequences in other domains

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The time-saving bias is not limited to driving. The same faulty estimations emerge when people are asked to estimate savings in patients’ waiting time when adding more physicians to a health care center[9] or when estimating an increase in the productivity of a manufacturing line by adding more workers.[10]

## See also

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* List of cognitive biases
* Amdahl's law

## References

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1. ^ Svenson, Ola (1970). "A functional measurement approach to intuitive estimation as exemplified by estimated time savings". Journal of Experimental Psychology. 86 (2): 204–210\. doi:10.1037/h0029934 – via American Psychological Association.
2. ^ a b Peer 2010a.
3. ^ Svenson 2008, Experiment 1.
4. ^ a b Fuller et al. 2009.
5. ^ Fuller et al. 2009, p. 14.
6. ^ Svenson 2008.
7. ^ Peer 2010b, Study 3.
8. ^ Peer 2011.
9. ^ Svenson 2008, Experiment 2.
10. ^ Svenson 2011.

### Sources

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* Fuller, R.; Gormley, M.; Stradling, S.; Broughton, P.; Kinnear, N.; O'Dolan, C.; Hannigan, B. (2009). "Impact of speed change on estimated journey time: Failure of drivers to appreciate relevance of initial speed". Accident Analysis & Prevention. 41 (1). Elsevier BV: 10–14\. doi:10.1016/j.aap.2008.07.013. ISSN 0001-4575. PMID 19114132.
* Peer, E. (2011). "The time-saving bias, speed choices and driving behavior". Transportation Research Part F: Traffic Psychology and Behaviour. 14 (6). Elsevier BV: 543–554\. Bibcode:2011TRPF...14..543P. doi:10.1016/j.trf.2011.06.004. ISSN 1369-8478.
* Peer, E. (November 2010a). "Speeding and the time-saving bias: how drivers' estimations of time saved in higher speed affects their choice of speed". Accident Analysis & Prevention. 42 (6): 1978–1982\. doi:10.1016/j.aap.2010.06.003. ISSN 1879-2057. PMID 20728651.
* Peer, E. (December 2010b). "Exploring the time-saving bias: How drivers misestimate time saved when increasing speed". Judgment and Decision Making. 5 (7): 477–488\. doi:10.1017/S1930297500001649. S2CID 18481243.
* Svenson, O. (2008). "Decisions among time saving options: When intuition is strong and wrong". Acta Psychologica. 127 (2). Elsevier BV: 501–509\. doi:10.1016/j.actpsy.2007.09.003. ISSN 0001-6918. PMID 17936234.
* Svenson, O. (2011). "Biased decisions concerning productivity increase options". Journal of Economic Psychology. 32 (3). Elsevier BV: 440–445\. doi:10.1016/j.joep.2011.03.005. ISSN 0167-4870.

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