Calibration Certificate No. time-saving-bias

Time-Saving Bias

People systematically misestimate time saved by speed increases: they overestimate savings when accelerating from already-high speeds and underestimate savings when accelerating from low speeds.

systematic

documented offset from the normative answer

Status — robustgrounding ± strong

The time-saving bias has been replicated consistently across multiple countries, paradigms, and populations over more than 50 years. It holds in questionnaire studies, driving simulators, and field studies with taxi drivers. The effect direction is never reversed. A 2010 study by Peer called it 'a genuine, systemic and robust cognitive bias' after finding it across eight different question variants with no moderating effects of gender, age, or driving experience.

Calibration record

Intuitive time-saving judgments questionnairenot reportedParticipants underestimated time savings when initial speed was low (~45 kph or less) and overestimated time savings when initial speed was high (~70 kph or more). Approximately 90% of responses were better fit by a proportion heuristic than the correct formula. — Svenson, 1970 · n = 56 university students

Model engine speed adjustment tasknot reportedWhen increasing from low speeds, participants consistently overshot the required speed, driving faster than necessary. This demonstrated the bias extends beyond hypothetical judgments to active motor control with monetary incentives. — Svenson, 1973 · not reported in abstract

Driving simulator time-saving tasknot reportedWith standard speedometers, drivers saved more than 3 minutes when starting from 30 kph (mean ~6.5 minutes saved) and less than 3 minutes from 100 kph (mean ~2 minutes saved), confirming the bias in active driving. The inverted-speed meter condition reduced the bias significantly. — Eriksson, Svenson & Eriksson, 2013 · n = 24 drivers per condition

Source of systematic error

The time-saving bias arises because people use a proportion heuristic, estimating time saved based mainly on the proportional speed increase (V2-V1)/V2 rather than the correct inverse relationship. People neglect the critical role of initial speed: doubling speed from 30 to 60 kph halves journey time, but doubling from 70 to 140 kph saves far less proportionally. The heuristic is cognitively easier than computing the correct formula, which requires understanding that speed is a ratio of distance over time, not a directly additive quantity.

Some researchers argue the bias reflects a fundamental failure to understand that speed is a derived measure (distance/time) that cannot be concatenated like distance or time itself. Others emphasize the neglect of initial speed as the primary source of error.

Recalibration procedure

Present the inverted speed (time per unit distance, e.g., minutes per mile) rather than speed in distance per time. When drivers see minutes per mile on a speedometer, their time-saving judgments become significantly more accurate.

Eriksson et al. (2013, 2015) showed that participants with an inverted-speed meter drove speeds that saved closer to the target 3 minutes compared to controls with standard speedometers, effectively eliminating the bias.

Cross-calibrated against

MPG IllusionsiblingThe MPG illusion, identified by Larrick & Soll (2008), involves parallel errors in judging fuel efficiency: people think the gas saved by switching from 10 to 15 MPG is less than switching from 30 to 40 MPG, when the opposite is true. Both biases stem from ratio neglect.
Proportion dominance heuristicparentThe time-saving bias exemplifies a broader tendency to judge ratio quantities by their proportional change rather than their reciprocal relationships.

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