The influence of weather conditions on road safety

The annual number of road crash casualties fluctuates due to chance, mobility changes, changes in road safety measures, and other influencing factors, such as weather conditions. In order to better understand these fluctuations in road crash casualties, and potentially attribute them to changes in road safety measures or implementations, it is necessary to know how every factor relates to the changes in the number of crashes and casualties. The influence of changes in extreme weather conditions is often identified as a cause of fluctuations in road safety and the resulting numbers of crashes and casualties (see e.g. SWOV Fact sheet The influence of weather on road safety). Understanding the influence of the weather on these fluctuations is necessary when the aim is to reliably estimate the effects of safety measures. Furthermore, it may be possible to develop or enhance specific safety policies to help counter adverse effects of weather conditions on road safety. The topic of the influence of weather conditions on road safety has resulted in many studies published in international journals and technical reports, a number of which are reviewed in the SWOV fact sheet The influence of weather on road safety. There is one problem in applying the results obtained from studies performed in other countries than the Netherlands to the Dutch road safety situation: the general climate in other countries can have an impact on how road users, legislators and road authorities react to (changes in) weather conditions. In mountain areas or in Nordic countries road authorities and drivers are better prepared for snow than, for instance, in the Netherlands. Differences in the mix of modes of transport could also be a factor: the role of bicycles and pedestrians in the Netherlands may be quite different from that in other countries. These factors may also affect the impact weather conditions have on road safety. Therefore, the available literature can mainly be used for guidance on the methodology on how to measure the effects of weather conditions on road safety in the Netherlands. Without modifications the results cannot easily be applied to Dutch circumstances. The effects of weather conditions on road safety can be studied using different approaches. It is to be expected that weather conditions influence both road travel and the risk of road travel. One approach towards analysis of the effects of weather conditions on road safety is to analyse both effects separately and to combine these effects to obtain a general effect. An advantage of this approach is that the effects of weather conditions are decomposed into separate effects of mobility and risk, which may be interpreted separately. A disadvantage of this approach is that it requires information on travel under different weather conditions, or it must be assumed that travel is not affected by weather conditions. Very limited information on travel under different weather conditions is available in the Netherlands, as this requires to match both travel data and weather data. In addition, it appears reasonable to assume that for many modes of transport the distance travelled is usually affected by weather conditions. Therefore, this report focuses on an analysis of the aggregate, accumulated effect of weather conditions on the number of road crashes and injuries. This approach has the advantage of being applicable to most modes of transport because it makes no assumptions on how weather conditions affect mobility (distance travelled). It has however the disadvantage of its inability to determine the separate effects of weather conditions on mobility and risk. Based of the experiences published by Eisenberg (2004), it was decided to analyse data at the daily and national aggregation level. Data for this level is available for the Netherlands. One analysis was performed relating the daily aggregated weather conditions to daily aggregated travel and road casualty data. The results indicate that it is mostly non-professional travel (determined by reported trip motive) that is affected by weather conditions. Furthermore, but to a lesser extent, professional travel by weather sensitive travel modes, such as bicycle and moped, appear to be influenced by weather conditions. Further analysis was conducted analysing the effects on the number of casualties in the Netherlands of the duration of precipitation, which the first analysis identified as an important factor. An approximate model was developed for this purpose. The model is based on the daily numbers of casualties under both dry and wet weather conditions (which are modelled simultaneously) and is used to determine the effects of precipitation under various combinations of conditions, including the length of the period considered (longest period 1987-2007), season, road user type and crash opponent type. This approach allows to reliably estimate the relative effect of precipitation on road safety. The main benefit of this approach is that it relates 'precipitation crashes and casualties to the duration of the precipitation and 'dry crashes and casualties to the duration of the dry spells. Models that relate the total number of crashes or casualties to, for example, the amount of precipitation, cannot determine whether more crashes coinciding with more precipitation are in fact crashes caused by that precipitation. This issue becomes even more important when rarer weather phenomena than precipitation are studied. Even wet weather conditions occur only about 7% of the time in the Netherlands. Care should be taken in using an explanatory variable for the entire period which is not relevant for 93% of the time. The method presented in this report can only be used when the relevant weather conditions can be determined for the casualty data. The weather condition also has to be dichotomous; it has to be either present or absent. In practice this limits the applicability of this approach for determining the effect of precipitation. Two important tendencies in results can be distinguished for road safety in the Netherlands: the effects are different for different levels of crash severity and the effects are different for vulnerable transport modes and less vulnerable transport modes. Furthermore, it appears that in the Netherlands the impact of precipitation is more severe during the winter months than in other periods (snow and ice are sufficiently rare in the Netherlands to tentatively eliminate snow as an explanation). Apart from seasonal differences there is some indication that the effect of precipitation on the number of fatalities decreases over the years. Finally, it should be mentioned that the results found in this study do not easily translate into potential road safety measures designed to limit the adverse consequences of weather. There is no clear aspect on which attention can be focussed.