The number of seriously injured road users is used both as an indicator of traffic safety trends in Sweden and in more specific studies of, for example, different road user groups.
A seriously injured person is defined as a person who suffers injuries that lead to permanent medical impairment of at least 1 percent. The term very seriously injured is used as a complement and is defined correspondingly but refers to injuries that lead to permanent medical impairment of at least 10 percent.
In practice, the level of impairment is not known at the time of the accident, and therefore the number of seriously and very seriously injured persons are forecasted. The basis for the forecasts are Strada, the national database for road traffic accidents in Sweden, and a method developed by Folksam (a Swedish insurance company) that estimates the risk that a person’s injuries will lead to permanent medical impairment in the future.
The aim of this study is to present the size of the uncertainty of the forecasts for different subpopulations such as geographical areas, roads user groups, age and gender.
The uncertainty of the forecasts is illustrated as forecasting intervals. It became clear during the study that derivation of the intervals was relatively easy for the case with one injury per person but considerably more difficult when the persons had a combination of several injuries. Therefore, the width of the intervals was determined using simulation instead of theoretical calculations.
The estimated number of seriously injured road users (excluding pedestrians who have fallen) in Sweden 2014 was 4 744 ± 5 %. The corresponding number of very seriously injured was 689 ± 14 %. The relative uncertainty of the forecasts increases when the data is divided into subpopulations. For example, the number of seriously injured cyclists during 2014 was estimated as 2 094 ± 7 % and the number of seriously injured persons in a truck was 52 ± 20 %. Note that the results have not been corrected to compensate for the fact that not all emergency hospitals in Sweden were reporting to Strada in 2014. The uncertainty is decreased if the forecasts instead are based on three years of data but are still large for small subpopulations.
This report shows forecasting intervals for populations divided by gender, age, road user, counties and a selection of municipalities. These intervals can be used as guidance also for other subpopulations, for example other municipalities. However, the width of the intervals does not only depend on the number of injured persons in the population but also other factors such as the type of injury. The intervals may therefore differ between populations that are comparable with respect to size.