Effects of roundabouts on road casualties in the Netherlands

Roundabouts have been increasing in number in the Netherlands since the 1980s, and the number of roundabouts has more than doubled since 1998. The application of roundabouts in road design was particularly strong during the Sustainable Safety Start-up Programme, which started in 1997 and continued until 2002, when many intersections were reconstructed into roundabouts. In the literature, the effects of reconstructing intersections into roundabouts on the number of crashes on those locations have been documented in before and after studies and, although the reported effects vary, the overall reduction in crashes is significant. However, the question remains: did roundabouts contribute to the improvement of road safety in the Netherlands? This study evaluates the effects on road crash casualties and takes into consideration all crashes on all known roundabouts built in the Netherlands during the period of 1999 to 2005, not just a sample. Before and after crash and roundabout information is used and specific attention is paid to fatalities and (police reported) inpatients. A cross-section comparison of road junctions and roundabouts is also made. The locations of roundabouts are stored in a database of the road network in the Netherlands which is updated yearly. Linking construction year of individual roundabouts and before and after traffic casualties at these locations is achieved through the use of a Geographic Information System (GIS). Crashes in the before period were selected based on their geographical location, within a certain distance from the roundabout. Several checks and improvements of the data have been undertaken to achieve the best possible quality of the data used for the analysis. In some cases, the location of the original intersection (if existent at all) need not be within that area. This causes the number of casualties in the period before the construction of roundabouts to be somewhat underestimated and therefore results will be biased towards unfavourable effects for roundabouts. Study findings are consistent with other studies. However, the data set and method applied result in statistically significant effects for aggregated fatal casualties not previously obtained in the literature, even though the method is biased towards the opposite. Fatality data aggregation from a large data set allows the application of conventional before and after tests to fatal casualties which are rare and random occurrences, particularly when small samples of intersections are selected. The effect on the number of hospitalized casualties and all casualties is also examined and the applicability and opportunities of this method to other studies is discussed. As far as interpretation of the results for casualties is concerned, care has to be taken, as underreporting of casualties, especially for non-motorized vehicle crashes, is a serious problem. Although a before and after approach was taken, the values determined are general in nature due to the aggregation method, and aggregation of all 2,009 roundabouts implemented between 1999 and 2005. The results are not to be used as predictive values, but are rather intended as average summary values. In other words, the aggregate road safety consequences of the reconstruction of all intersections in the Netherlands that were converted into roundabouts has been evaluated. This was done instead of making a new estimate of the general effectiveness of the reconstruction of an intersection into a roundabout. It seems that roundabouts built from 1999 to 2005 have been effective for road safety in the Netherlands, resulting in a reduction of about 76% in fatal casualties and about 46% in (police reported) killed or hospitalized casualties. If these results hold, about 12 lives were saved and 102 killed or hospitalized casualties were prevented in 2006.