no77 2009
An Approach to Safe Driving for Elders 
Can accidents be avoided thanks to a passenger?
  Most people who have just obtained their driver's license probably start out with an experienced driver sitting in the passenger seat. As they get used to driving, however, they are likely to feel annoyed at their partner's advice; some elderly couples may even start arguing while driving.

  The influence of passengers has been studied and reported by ITARDA (1) and many foreign researchers. In a Swedish research study (2) using data on travel distance by different number of passengers, the presence of passengers is reported to contribute to the prevention of traffic accidents in Sweden, and the preventive effect is reported to increase with the increase in the number of passengers.
Based on the examination of a number of European and American studies on this subject, this paper also introduces both positive (many European countries) and negative (U.S., Australia, etc.) effects of the presence of passengers on the prevention of traffic accidents and concludes that this difference is attributable, among other factors, to the data collection methods adopted, regional differences, and the minimum age for obtaining a driver's license (16 or 18).

  In this issue of ITARDA Information, we discuss the influence of passengers, based on recent Japanese reports(3) , (4) on this subject.

1. Introduction
2. Traffic accidents when driving with passengers
3. Presence of passengers and reduced accident rate
4. Preventive effect of passengers on traffic accidents
5. Conclusion

Appendix: Correlation between passengers and traffic accidents

- Does ride sharing cause more casualties?

Even if we assume that all the occupants will be killed or injured in an accident, it is highly significant to point out that the relative accident rate with one passenger is less than half of that with no passengers, as shown in Fig. 4.

For example, the relative accident rate for a male driver over 75 years old driving alone during the daytime is 14, but it drops to 4.5 if one passenger is present in the car.
Comparing the number of expected casualties in accident cases with and without passengers as follows, fewer people are expected to be killed or injured in the case of driving with one passenger:

Number of expected casualties without passengers > Number of expected casualties with one passenger
(1 person × 14 > 2 people × 4.5)

Therefore, driving with one passenger does not cause more casualties.

Concerning the question of whether it is safer for the passenger to sit in the front seat or the rear seat, further studies are needed taking into consideration various aspects such as the greater preventive effect of the front-seat passenger than that of the rear-seat passenger on the one hand and the higher fatality rate for front-seat passengers (6) on the other.

- Front-seat passenger vs rear-seat passenger

Although the foreign research study mentioned earlier (2) reveals that the preventive effect increases with the increase in the number of passengers, this tendency was not observed in the Japanese data used for the present analysis even when two or more passengers are with the driver.

This difference is attributable to the fact that a common type of accident in Europe and the U.S. is one in which a car does not involve other vehicles or pedestrians, whereas vehicle-to-vehicle and person-to-vehicle collisions constitute the major types of accident in Japan.

We have explained the significant effect of the presence of passengers especially in recognizing outside objects and perceiving danger, but this effect is not considered to increase even if the number of passengers increases, considering the seating position of passengers. That is, the first passenger in private driving is most likely to sit in the front seat (Table 1), with the result that the second and following passenger(s) sit in the rear seat and do not have a clear front view.

However, the presence of multiple passengers may contribute to accident prevention more effectively if the situation requires additional support from passengers for safe steering and appropriate judgment rather than recognition of outside objects that could be hit.

Institute for Traffic Accident Research and Data Analysis (ITARDA)