With millions of fans driving to stadiums every week, is it more dangerous to take to the road after a football game?

Football and cars just seem to go together. The entire tradition of tailgating is based on the assumption that fans will pile their coolers and grills into massive vehicles and, along with several thousand other auto-loving fans, make the trek to the stadium.

Just in time for the NFL season kickoff next Thursday, "Football Game-Day Fatalities," a map and analysis by IDV Solutions's John Nelson, takes a look at one of the more sobering effects of this connection, comparing the number of traffic fatalities in a given city on days with NFL home games and days with away games for ten seasons, from 2001 to 2010. The analysis and map are based on local drive time data, team schedules, and traffic fatality data from the National Highway Transportation Safety Administration.

Previous studies looking at the relationship between football and car crashes have found that traffic fatalities tend to spike after the Super Bowl. These researchers generally suggest that the surge in fatalities is due to both drinking and driver fatigue. In this map, Nelson wanted to look more specifically at the effects of a high-attendance event on the cities where the teams are actually playing.

The map that Nelson and cartographer Josh Stevens put together is below. Each yellow dot represents a traffic fatality on the day of an NFL home game, while each white dot stands for a death on the day of an NFL away game. The usual caveats about not assuming causation apply fairly obviously in this case. In quite a few cities, the number of traffic deaths was, thankfully, just too low to even be statistically significant. And, you'll notice that some of the country's bigger metro areas — San Francisco, New York, and Baltimore-Washington — aren't included because there was no easy way to tease out the effects of having two NFL teams so close together.

John Nelson and Josh Stevens.
The big overall takeaway is that Nelson found no association between NFL home games and traffic fatalities for the ten-year period he studied. He notes that this surprised him, as his initial hypothesis was the "intuitive notion" that home games would correlate to a higher number of fatal accidents, especially in places where the stadium is far from the city and the weather is bad.
Nelson is also careful to point out the limits of the analysis. It is effectively impossible to compare these game-day traffic deaths to either the overall number of drivers on the road, or the number of car crash fatalities on days with no football game at all. Instead, the map looks at a simple variable: is the game played at home, or somewhere else? Nelson explains that the map "does not imply that a football game has anything to with any individual traffic fatality, and over the course of 80 home game-days and 80 away game-days most locations tended to be generally even (to my initial surprise, given the tens of thousands of vehicles making a dedicated pilgrimage)."

There are of course fairly compelling reasons for Nelson's overall finding that there was no relationship between game location and traffic fatalities. As he notes, home games take tens of thousands of drivers off the roads for several hours during each home game. That may well cancel out the effects of higher crash rates after the game, when intoxicated and tired fans hit the road again en masse. While accidents may go up overall, fatal accidents are less likely when traffic is moving slowly.

However, Nelson did find substantial local variation by city. He summarized his city-by-city findings for game day accidents in the areas within a one-hour drive of NFL stadiums in the chart below.

He found statistically significant differences (to 99 percent) for five different metro areas. Two cities saw overwhelmingly more traffic deaths on their home-game days: Dallas (72.3 percent) and Seattle (84.2 percent). On the other hand, three regions had far more deadly days when their football team was on the road: Denver (72.2 percent), New England (67.9 percent), and Tampa Bay (66.7 percent).

Nelson is quick not to jump to any conclusions for these curious trends, and cautions that he looked at only one simple variable without assuming causation. After all, there's no way to tell if, unlikely as it is, there are actually jersey-bedecked fans getting caught in these horrific accidents.

What then might account for these differences? What kinds of factors might separate out Dallas and Seattle, with high home-game fatalities, from Denver, New England, and Tampa, with much lower ones? Traffic researchers note that several factors tend to correlate with more and deadlier accidents: speed of traffic and speed limits, weather, driver fatigue and time of day, distracted drivers, and of course, alcohol.

Still, Nelson's results present an interesting mix. Dallas and New England, on opposite ends of the traffic-fatality spectrum, both have stadiums far from the center city. Curiously, Denver and New England both have far worse winter weather than Dallas, yet those cities saw more fatalities on days without tens of thousands of fans trekking to the local stadium. Or maybe it's the size of the traffic flow, relative to the total population. Places like L.A. or Chicago are big metros, compared to, say, Green Bay, so the overall effect on traffic patterns would be smaller. One argument, using data from Patrick Adler, an urban planning doctoral student at UCLA, could be that the effects of a home game might be magnified in Green Bay, where the 70,000+ average attendance represents nearly a quarter of the metro area's population. But Cowboys and Patriots games both drew out about 2.5 percent of the area's population (2.59 and 2.31 percent, respectively). So that's not an answer either.

Nelson's research represents a jumping off point, more than anything. He's put together a good data set to better understand the relationship between traffic data and large-scale sports events. Now, future studies can add in factors like population size, weather conditions, driving distance, stadium location, speed limits, local drunk driving laws, and other controls.

It seems fairly clear that being a dedicated Cowboys fan and a diehard Patriots fan provide two very different experiences, and maybe having a safe drive home is just another to add to the list. But the big takeaway—and a contrast from other studies—is that there appears to be no relationship between large-scale sporting events and traffic fatalities overall, a counter-intuitive finding that begs for more research on the question.

Top Image: A boy throws a football in the parking lot at the New Meadowlands Stadium on August 16, 2010. REUTERS/Lucas Jackson.

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