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Update motorcycle post for brevity

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agude committed Feb 21, 2019
1 parent 6776435 commit b0d5f8010df6f0d419bf1c3f36409f8a16165fc4
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  1. +56 −58 _posts/2017-02-21-switrs_motorcycle_crashes_by_date.md
@@ -17,9 +17,9 @@ categories: switrs

{% include lead_image.html %}

A few months ago I wrote a post in which [I explored when accidents happen in
California][lastpost]. This time I'm going to go through the same analysis but
restrict myself to looking at accidents involving motorcycles. Motorcycle
A few months ago I wrote a post in which [I explored when car accidents happen
in California][lastpost]. 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][switrs] data: my
father rode motorcycles for years (he only recently stopped) and we wanted to
better understand what sort of risks that brought.
@@ -28,10 +28,10 @@ better understand what sort of risks that brought.
[switrs]: http://iswitrs.chp.ca.gov/Reports/jsp/userLogin.jsp

I expected the accident trend for motorcycles to match the one I found when
[looking at cars][lastpost].[^1] 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 this pattern.
[looking at cars][lastpost]. 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
this pattern.

One thing before we get started: the number of riders on a given day (or more
accurately, the [number of miles ridden by them][vmot]) has the most impact on
@@ -74,12 +74,12 @@ not yet complete data for the year.

## Accidents per Week

For cars, [I found that there was a decrease in accidents][apw]
starting in 2008 as people stopped driving to work during the [Great
Recession][gr]; 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.
For cars, [I found that there was a decrease in accidents][apw] starting in
2008 as people stopped driving to work during the [Great Recession][gr]. Apart
from that, 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 a similar pattern. However, the trends (plotted
below) are completely different.

[apw]: {% post_url 2016-12-02-switrs_crashes_by_date %}#accidents-per-week
[gr]: https://en.wikipedia.org/wiki/Great_Recession
@@ -89,31 +89,28 @@ trends. However, the trends (plotted below) are completely different.

[per_week_plot]: {{ file_dir }}/motorcycle_accidents_per_week_in_california.svg

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][dot]. The next observation is that, unlike for
cars, there is a strong seasonal effect---even in sunny California, motorcycle
ridership drops drastically in the winter! And unlike the trend for cars,
there is not a large decrease due to the recession. Finally, I note that there
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][dot]. There is also a strong seasonal effect---even in sunny
California, motorcycle ridership drops drastically in the winter! And unlike
for cars, there is not a large decrease due to the recession. Finally, there
is an overall upward trend in the number of motorcycle accidents.

[dot]: https://www.fhwa.dot.gov/policyinformation/statistics/2012/mv1.cfm

As I noted above, commute accidents for cars 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 cars, 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.
Commute accidents account for the majority of car accidents. However, this
does not appear to be the case for motorcycles, because the numbers were
relatively unchanged by the Great Recession; people kept riding at the same
levels when out of work.

## Day-by-Day

When looking at car accidents [I observed that holidays were the maxima and
minima][dbd] 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.
For car accidents, [the days with the most accidents and least accidents were
holidays][dbd]. The largest number of accidents were on holidays where people
went to work and then out afterwards, like Halloween. Motorcycle accidents do
not follow that pattern. Instead, the holidays show quite disparate results:
some holidays dip, some spike, others show almost no deviation from a normal
day.

[dbd]: {% post_url 2016-12-02-switrs_crashes_by_date %}#day-by-day

@@ -124,25 +121,26 @@ year][average_accidents]][average_accidents]

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.
this as due to the seasonal weather:

There is one outlier that I must address. The sharp peak between Washington's
Birthday and St. Patrick's Day is [leap day][leapday]. This peak is a
statistical artifact. The mean for all other days is calculated with `n = 15`,
but only `n = 3` for leap day.
- In summer, any day is a good day to ride.
- In the winter, the weather keeps riders off the road, except when a holiday
gives them the extra motivation they need.

One final outlier to address: the sharp peak at the end of February is [leap
day][leapday]. The peak not an error, but is a statistical artifact. The mean
for all other days is calculated with `n = 15`, but only `n = 3` for leap day.

[leapday]: https://en.wikipedia.org/wiki/February_29

## Day of the Week

The weekends [showed a decrease in the number of car accidents][dotw]. 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
Car accidents [happen less during the weekend, when people aren't
commuting][dotw].

For motorcycles the weekends are prime riding times, and so the number of
accidents increases. 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][this_dbd]. 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
@@ -165,25 +163,25 @@ winter][accident_violin_plot]][accident_violin_plot]
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.
compared with summer (bottom row). 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, just as we expected. 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.
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 cars. 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.
continue riding even when they do not have a job to commute to. Seasons have
a large effect on the number of riders out on the road. Motorcycle ridership
has 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.

---

[^1]: Not just cars, but motorcycles, bicycles, pedestrians, etc. as well. However, it is mostly "cars" and so for simplicity that's what I'll refer to it as.
**Update**: _Edited post for brevity, clarity, and correctness._

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