Estimated time of arrival and debiasing the time saving bias.

The time saving bias predicts that the time saved when increasing speed from a high speed is overestimated, and underestimated when increasing speed from a slow speed. In a questionnaire, time saving judgements were investigated when information of estimated time to arrival was provided. In an active driving task, an alternative meter indicating the inverted speed was used to debias judgements. The simulated task was to first drive a distance at a given speed, and then drive the same distance again at the speed the driver judged was required to gain exactly 3 min in travel time compared with the first drive. A control group performed the same task with a speedometer and saved less than the targeted 3 min when increasing speed from a high speed, and more than 3 min when increasing from a low speed. Participants in the alternative meter condition were closer to the target. The two studies corroborate a time saving bias and show that biased intuitive judgements can be debiased by displaying the inverted speed. Practitioner Summary: Previous studies have shown a cognitive bias in judgements of the time saved by increasing speed. This simulator study aims to improve driver judgements by introducing a speedometer indicating the inverted speed in active driving. The results show that the bias can be reduced by presenting the inverted speed and this finding can be used when designing in-car information systems.

Debiasing overoptimistic beliefs about braking capacity.

We investigated, using questionnaires, different strategies for removing drivers' overoptimism (Svenson et al., 2012a) about how fast their speed could be decreased when they were speeding compared with braking at the speed limit speed. Three different learning groups and a control group made collision speed judgments. The first learning group had the distance a car travels during a driver's reaction time for each problem. The second group had this information and also feedback after each judgment (correct speed). The third group judged collision speed but also braking distance and received correct facts after each problem. The control group had no information at all about reaction time and the distance traveled during that time. The results suggested the following rank order from poor to improved performance: control, group 1, group 3 and group 2 indicating that information about distance driven during a driver's reaction time improved collision speed judgments and that adding stopping distance information did not add to this improvement.

Braking from different speeds: judgments of collision speed if a car does not stop in time.

The purpose of speed limits is to keep driving speed low enough for drivers to be able to pay attention to relevant information and timely execute maneuvers so that the car can be driven in a safe way and stopped in time. If a driver violates a speed limit or drives too fast she or he will not be able to stop as quickly as from a slower speed. We asked participants to imagine that they themselves had driven a car outside a school at a speed of 30 km/h when a child suddenly had rushed into the street. From this speed it was possible to stop the car just in front of the child after braking as quickly and forcefully as possible. We then asked the participants to imagine that they drove the same street at a higher speed of 50 km/h and the child appeared at the same place as before. At what speed would the car hit the child after braking in the same way as before? This kind of problems were presented in three studies and the results showed that the judged speeds of collision were always underestimated in different hypothetical driving context scenarios by judges differing in numerical skills. This indicates an overly optimistic view on the possibilities to reduce speed quickly if the driving speed is too fast, which is an important component of attitudes towards speed limits, their legitimacy and recommended driving speeds. Further implications of the results were discussed last.