Independence Day

A frequently asked question is “How much more dangerous is travel over the Independence Day holiday?” Two aspects of this question must be considered: “Compared to what?” and “What about changes in the amount of driving?”

NSC compares the holiday to periods of similar length, specifically from 6 p.m. Tuesday to 11:59 p.m. Wednesday, during the weeks immediately before and after the Independence Day holiday. This chart shows the fatality data from FARS for 1996 to 2016 for comparable holidays. The average number of traffic deaths during Independence Day over the last five 1.25-day holiday periods is 41.6% higher than the average number of traffic deaths during the comparison periods (185 vs. 131 deaths). The difference between these two means is statistically significant at the .05 level.

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  • Data Table

Source: NSC analysis of NHTSA FARS data

NSC also compares the Independence Day holiday to other holiday periods. When comparing holiday periods of different lengths, an average fatality per day rate is used. The Independence Day holiday, along with the other summer holidays (Memorial Day and Labor Day) have the highest average fatality rates per day. Thanksgiving, Christmas, and New Year’s Day holidays tend to have lower average fatality rates per day.

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  • Data Table

Source: NSC analysis of NHTSA FARS data

The second question concerns changes in the amount of travel or exposure. NSC is not aware of any data system that tracks changes in vehicle miles of travel by day of the year on a national basis. Lacking an objective measure of exposure change, NSC assumes travel is greater on holidays than on equivalent non-holiday.

If the assumed travel increase exceeds 41.6%, then the risk of dying in a traffic crash during the Independence Day holiday period is less than during comparable non-holiday periods. If the travel increase is less than 41.6% or if travel is actually lower, the risk of dying on the holiday is greater than during comparable periods.

Estimate methods

The objective is to estimate the number of deaths that will occur in traffic crashes during the Independence Day holiday period based on data available several weeks before the holiday. The estimate developed by NSC includes all traffic deaths from crashes during the holiday period.

The general procedure involves three steps:

  • Historical data are used to determine the average fraction that holiday fatalities are of the total motor-vehicle deaths for the month
  • Total traffic deaths for the coming month in which the holiday falls are estimated using a time series forecasting model
  • The projected total for the month is multiplied by the fraction to obtain the holiday estimate.

Holiday as percent of monthly fatalities: Total July motor vehicle deaths are calculated using the latest four years of final data available from the National Center for Health Statistics. Traffic fatality estimates for the Independence Day period are calculated using data from NHTSA FARS.

The table shows total motor vehicle fatalities for July and traffic fatalities from crashes that occurred during the holiday period. For the previous five 1.25-day holiday periods, fatalities from crashes during the Independence Day holiday period averaged 4.59% of the total fatalities in July.

Traffic deaths during the Independence Day period as a percent of total motor-vehicle deaths in July

Year June Independence Day Period Percent
1984 4,470 178 3.98%
1990 4,430 232 5.24%
2001 3,920 173 4.41%
2007 3,980 184 4.62%
2012 3,340 157 4.70%
5-year average 4,028 185 4.59%

Source: Accident Facts, Injury Facts, and NHTSA FARS data

A time series model was developed to forecast an estimate of total traffic deaths for July 2018. An Autoregressive Integrated Moving Average model was constructed based on 48 months of traffic deaths recorded from May 2014 through April 2018. This 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 July 2018 to be 3,562. Multiplying the projected total fatalities by the fraction obtained in the first step gives an estimate of 164 traffic fatalities from crashes during the holiday period.

The 90% confidence interval for the estimate of total July motor vehicle deaths is 3,264 to 3,886. If we assume the fraction of July deaths that occur during the Independence Day period is normally distributed, then the 90% confidence interval for that fraction is 4.18% to 5.00%. Combining these two estimates gives the confidence interval for the Independence Day period estimate: 136 to 194 traffic deaths.