Modelling Weather Effects on Road Casualty Statistics
Daryl Lloyd, Department for Transport, David Mais, Department for Transport, Jennifer Davies, Office for National Statistics
Short term weather and climate changes can have significant effect on the number of road traffic casualties. This work quantifies the effect precipitation and temperature has on casualties in Britain.
The Department for Transport publish statistics on the number of people killed and injured in road accidents in Great Britain that are reported to the police. It has long been known that the weather impacts on year-on-year changes in these figures. It is important for road safety policy making to be able to assess how much the weather has contributed to these changes.
A cross government group looking at how the weather impacts on different types of statistical series can be assessed was established. This led to the development of a statistical model to produce a weather-adjusted road casualty series. The work undertaken uses a regARIMA modelling approach (time series modelling with a regression) that has been used. The modelling makes use monthly precipitation and average temperature data at a national level and how far these deviate from the 30-year long term average.
As a result of the model, the Department for Transport now regular produces a ‘weather-adjusted’ time series. This series is not to replace the key casualty figures, but providers road safety practitioners with a guide to how much of the annual changes in casualty figure is down to weather conditions. Once these effects have been removed the final adjusted series gives a better indication of road safety performance in terms of other influencing factors.
As an example, there was a large and unexpected increase in road deaths in Great Britain between 2010 and 2011. However, when unusually bad weather at both the start and end of 2010 is accounted for this increase no longer exists.
Since the creation of the original model, the Department has continued to develop the model to look at regional patterns and daily deviations.
The model demonstrated can be transported to other countries.
Association for European Transport