Time-saving bias

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Time-saving bias is the tendency to misjudge how much time you save (or lose) when you go faster or slower. People often underestimate the time saved by speeding up from a low speed and overestimate the time saved by speeding up from a high speed. The same pattern appears when slowing down: they underestimate time lost from a low speed and overestimate time lost from a high speed.

In driving, this bias can lead to unrealistic speed choices. For example, people may favor a plan that raises average speed from 70 to 110 km/h over one that raises it from 30 to 40 km/h, even though the smaller increase saves more time. In questions about travel time, people often misjudge how much time would be saved on a 10-mile trip when changing from 30 to 40 mph versus starting at 60 mph.

The real relationship isn’t linear. Time saved from increasing speed is larger when you start slow and smaller when you’re already fast. For example, going from 20 to 30 mph saves about 10 minutes on a 10-mile trip, while going from 50 to 60 mph saves only about 2 minutes. People tend to use simple rules of thumb—like the percent change or the speed difference—so they get the actual savings wrong.

This bias isn’t limited to driving. It also shows up in other areas, such as estimating how much waiting time improves when more doctors are added, or how much productivity increases when more workers are hired.