SWITRS: On What Days Do Motorcycles Crash?

A black and white photo from 1959 of a policeman in full-leather protective gear. He stands over a motorcycle on side of a street in Stockholm as traffic passes by.

A few months ago I wrote a post in which I explored when accidents happen in California. This time I’m going to go through the same analysis but restrict myself to looking at accidents involving motorcycles. Motorcycle accidents are the original reason I tracked down the SWITRS data: my father rode motorcycles for years (he only recently stopped) and we wanted to better understand what sort of risks that brought.

I expected the general trends to match the trends I found when looking at all motor vehicles. There I found that commute accidents accounted for the majority of accidents, and so holidays and weekends that most people have off result in fewer accidents. Motorcycles, we will see, do not follow these trends.

One thing before we get started: the number of riders on a given day (or more accurately, the number of miles ridden by them) has the most impact on the number of crashes. If there are more riders, there are going to be more crashes. From now on I’ll treat the two numbers as equivalent, even though there are some confounding factors, like weather, which would change the ratio of accidents to number of riders on the road; I hope to look at these other factors in a later post.

As per usual, the Jupyter notebook used to perform this analysis can be found here (rendered on Github).

Data Selection

I selected accidents involving motorcycles from the SQLite database (discussed previously) with the following query:

SELECT Collision_Date FROM Collision
WHERE Collision_Date IS NOT NULL
AND Motorcycle_Collision == 1       -- Involves a motorcycle
AND Collision_Date <= '2015-12-31'  -- 2016 is incomplete

This gave me 193,336 data points (accidents) to examine spanning 2001 to 2015. Just as before, accidents from 2016 are rejected because there is not yet complete data for the year.

Accidents per Week

For all motor vehicles, I found that there was a decrease in accidents starting in 2008 as people stopped driving to work during the Great Recession; but apart from that trend, I found that the week-to-week rate changed relatively little, with holidays providing the largest increases and decreases. When I looked at just motorcycles, I expected to see similar trends. However, the trends (plotted below) are completely different.

Line plot showing accidents per week from 2001 to 2015

As expected, there are far fewer accidents because there are far fewer motorcycles: there are about 27 million vehicles in California, but of those only 770,000 are motorcycles. The next observation is that, unlike for all vehicles, there is a strong seasonal effect—even in sunny California, motorcycle ridership drops drastically in the winter! And unlike the trend for all vehicles, there is not a large decrease due to the recession. Finally, I note that there is an overall upward trend in the number of motorcycle accidents.

As I noted above, commute accidents for all vehicles account for the majority of accidents. However, the data suggest that for accidents involving motorcycles, commute traffic is not dominant. Moreover, unlike the results for all vehicles, people keep riding even when out of work; but they also stop riding when the weather is poor. Next we’ll look at accidents by day of the year instead of by week.

Day-by-Day

When looking at all motor-vehicle accidents I observed that holidays were the maxima and minima in the average number of crashes by day of the year. On holidays where people have the day off, the number of crashes decreases, whereas the number increases on holidays where people work and then go out afterward, like Halloween. Motorcycle accidents do not follow this trend. Instead, the holidays show quite disparate results: some holidays dip, some spike, others show almost no deviation from a normal day.

Line plot showing average motorcycle accidents by day of the year

The summer holidays do not stand out; only Memorial Day is readily visible. Winter holidays, by contrast, show both peaks and valleys. I would interpret this as due to the seasonal weather: during the summer, any day is a good day to ride; but during the winter the weather keeps many riders off the roads on most days. But it would appear that some winter holidays provide riders with the extra motivation to get out on the bike. Look, for example, at Martian Luther King Jr. Day, which occurs in January.

There is one outlier that I must address. The sharp peak between Washington’s Birthday and St. Patrick’s Day is leap day. This peak is a statistical artifact. The mean for all other days is calculated with n = 15, but only n = 3 for leap day.

Day of the Week

The weekends showed a decrease in the number of all motor-vehicle accidents. But for motorcycles, for whom weekends are the prime riding time, there is actually an increase on the weekends. If we think of weekends as a kind of mini-holiday, they provide a way to look at the same seasonal holiday phenomenon discussed above. Winter holidays showed high variance, so I would expect to see some weekends with high winter ridership, and some with low ridership. Summer holidays had low variance, so I expect to see similar ridership on all summer weekends.

The violin plots below show the distribution of accidents by day of the week over the 15 year period. They are divided into two seasons: summer (May–October) and winter (November–April).

Violin plot showing accidents by day of the week in summer and winter

There is lower ridership in winter over all (top row), as indicated by the central dotted line indicating average number of accidents. And we can see an increase on weekends; but during the winter, that weekend increase is small as compared with summer (bottom row). However, the winter distributions are more elongated than those from summer, meaning that on some days there are many riders, and on others there are almost none. Summer weekends, by contrast, have consistently high ridership.

Thus, it appears we can conclude that weekend rider behavior does seem to track seasonal holiday riding behavior. And like the trends for holidays, the weekend results could be due to weather.

Conclusion

Motorcycle accidents do not follow the same trends as for all motor vehicles. Motorcyclists continue riding even when they do not have a job. Seasons have a large effect on the number of riders out on the road. Riders are also out on holidays in the summer when other vehicles take the day off, and have high variance for winter holidays and weekends when the weather may turn against them. There are many more ways to explore motorcycle accidents—time of day, type of motorcycle, vehicle at fault—but those will have to wait for another day.