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Contents : Christmas Holiday Period Traffic Fatality Estimate 2008 Prepared by Kevin T. Fearn Statistics Department National Safety Council December 10 2008 Holiday period definition Christmas is December 25 and it is observed on that day. The length o f the holiday period varies depending on what day of the week December 25 falls. It may be 1.25 days long if December 25 falls on a Wednesday 3.25 days long if December 25 falls on a Friday Saturday Sunday or Monday or 4.25 days long if December 25 falls on a Tuesday or Thursday. In 2008 Christmas falls on Thursday so the holiday period is 4.25 days and extends from 6:00 p.m. Wednesday December 24 to 11:59 p.m. Sunday December 28.1 Method and results The objective is to estimate the number of deaths that will occur in traffic crashes during the Christmas holiday period based on data available several weeks before the holiday. The estimate developed here includes all traffic deaths from crashes that occur during the holiday period.2 The general procedure involves three steps. First historical data are used to determine the average fraction holiday fatalities are of total deaths for the month containing the holiday. Second total traffic deaths for the coming month in which the holiday falls are estimated using a time series forecasting model. Third the projected total for the month is multiplied by the fraction to obtain the holiday estimate. Holiday as percent of month. Total December deaths are the estimates published in Injury Facts (formerly Accident Facts ) two years after the year of the estimate. This figure is used rather than a revised estimate or the National Center for Health Statistics final count because it closely approximates the level of accuracy that the time series estimate will give for total monthly deaths in the current year. Fatality Analysis Reporting System (FARS) data were used to obtain deaths during the holiday periods. Table 1 shows the total traffic fatalities for the month of December and fatalities from crashes that occurred during the six most recent 4.25-day Christmas holiday periods. Fatalities from crashes during the Christma s holiday periods averaged 13.37% of the total fatalities in December. Time series model and projection. A time series model was developed to forecast an estimate of total traffic deaths for December 2008. An Autoregressive Integrated Moving Average (ARIMA) model was constructed based on 48 months of traffic deaths recorded from November 2004 through October 2008. An ARIMA model was chosen because of the seasonal pattern in traffic deaths. The model was developed using the SPSS/PC+ Version 5.0 statistical computer package. The model forecasts total traffic fatalities for December 2008 to be 3 228. Holiday estimate. Multiplying the projected total fatalities by the fraction obtained in the first step gives an estimate of 432 traffic fatalities from crashes during the holiday period. Confidence interval There is uncertainty associated with any estimate. The 90% confidence interval for the estimate of total December deaths is 2 970 to 3 509. If we assume that the fraction of December deaths that occur during the Christmas period is normally distributed then the 90% confidence interval for that fraction is 12.48% to 14.26%. Combining these two gives the confidence interval for the Christmas period estimate: 371 to 500 traffic deaths. 1 Nonfatal disabling injuries Based on the current disabling-injury to death ratio of 54:1 and rounded to the nearest hundred the estimate of the number of nonfatal disabling injuries that will result from crashes during the holiday period is 2 3 300 with a range of 20 000 to 27 000. Holiday comparison A frequently asked question is "How much more dangerous is travel over the Christmas holiday " There are two aspects of this question that must be considered. First compared to what And second what about changes in the amount of driving For most holidays we compare the holiday to periods of similar length before and after it. Because New Year's Day is exactly one week after Christmas we chose to compare Christmas to periods of similar length one week and two weeks before it. Specifically from 6:00 p.m. Friday to 11:59 p.m. Tuesday of the two weeks immediately before the Christmas holiday. Table 2 shows the fatality data from FARS for comparable weekends. The average number of traffic deaths during Christmas over those six years was 5.6% lower than the average number of traffic deaths during the comparison periods (506 vs. 536 deaths). The difference between these two means is however not statistically significant. The second question concerns changes in the amount of travel or exposure. We know of no data system that tracks changes in vehicle miles of travel by day of the year on a national basis. Lacking an objec tive measure of exposure change we assume that travel is greater on holidays than on nonholidays. If this in fact true then with greater travel

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