Greetings @witoldstupnicki
I have been looking at your work and something has caught my attention, check that what I say is true because I can be wrong. 
When you visualize the day and night graphs in cell 12, the axes refer on one side to the traffic volume and on the other side to the frequency. Seems odd.
If you make a histogram, which is what you do when you call plt.hist()
at the bottom, what you do is to apply a table of frequencies grouped thanks to the use of the series.
When you set the condition between day and night in cell 9, I would say that it does not work properly
This is what I did:
night_bool = (traffic['date_time'].dt.hour >=19) | (traffic['date_time'].dt.hour <=7)
horas_nighttime = traffic.loc[night_bool,'date_time'].dt.hour
horas_nighttime.unique()
array([19, 20, 21, 22, 23, 0, 1, 2, 3, 4, 5, 6, 7])
I hope I have not made a mistake.
I show you what I have done so that you have another point of view.
plt.figure(figsize = (15,4))
plt.subplot(1,2,1)
plt.hist(horas_daytime)
plt.xlabel('Day hours from 7 AM to 7 PM ')
plt.ylabel('Cars per hour')
plt.title('Traffic Volume: Day')
plt.subplot(1,2,2)
plt.hist(horas_nighttime)
plt.xlabel('Night hours from 7 PM to 7 AM')
plt.ylabel('Cars per hour')
plt.title('Traffic Volume: Night')
plt.show()
I send you my best regards and I hope I have helped you more than anything else.
A&E