Update motorcycle post for brevity

_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._