Learn Python Data Analytics by Example — Chicago Traffic Crashes | by Nick Cox | Towards Data Science

While working towards my Master’s in Business Analytics, I found that learning by example is the best way for me to learn Python data analytics. Being given a dataset and a set of coding tasks is much more beneficial than reading a textbook or listening to a professor.
I want to share this method of learning with others who will also benefit. All you need is a Python development environment (I recommend Jupyter Notebook) and a willingness to learn and have fun.
Included in this article is a list of data analytics tasks, followed by a detailed walkthrough of how to complete the tasks. Please try to complete the tasks yourself before reading through the walkthrough — you will get more out of it that way. Do keep in mind that there are many many ways to solve coding problems, so your code likely will not match mine word for word and that is okay.
Please also check out: Learn Python Data Analytics By Example: NYC Parking Violations and Learn Python Data Analytics By Example: Airline Arrival Delays.
For this project we will use a dataset with a combined nearly 1.5 million records, which represent traffic crashes reported in Chicago from 2013 to 2021 and the vehicles involved in them. The dataset was created in February 2021 and contains data sourced from the City of Chicago.
You will need to install the pandas, matplotlib, seaborn and Folium libraries, if you do not already have them.
Please perform the following tasks in Python using the crashes.csv and crashes_vehicles.csv datasets available from this drive. Also refer to this GitHub repo.
Read the CSV files containing the Chicago traffic crash data. Identify the column common to both files and merge them together on that column. Then display the total number of reported crashes.Change the ‘CRASH_DATE’ column to a date format. Drop observations that did not occur in 2018, 2019 or 2020 (other years have incomplete data).Display a plot showing the number of crashes that occur for each hour of the day.Name the make of vehicle that was involved in the most daylight crashes in August 2018. Remember that a crash can involve multiple vehicles.Determine which weather condition was most prevalent for each type of crash.Plot the primary contributing cause of reported crashes, from highest to lowest.Display the 10 state license plates involved in the most crashes. Remember that a crash can involve multiple vehicles.Display the proportion of crashes in each month of 2019 where alcohol was determined to be the primary contributing cause.Determine whether snowmobiles or recreational off-highway vehicles were involved in more crashes.Display a cluster map showing the locations of crashes involving a hit and run.
Read the CSV files containing the Chicago traffic crash data. Identify the column common to both files and merge them together on that column. Then display the total number of reported crashes.
Change the ‘CRASH_DATE’ column to a date format. Drop observations that did not occur in 2018, 2019 or 2020 (other years have incomplete data).
Display a plot showing the number of crashes that occur for each hour of the day.
Name the make of vehicle that was involved in the most daylight crashes in August 2018. Remember that a crash can involve multiple vehicles.
Determine which weather condition was most prevalent for each type of crash.
Plot the primary contributing cause of reported crashes, from highest to lowest.
Display the 10 state license plates involved in the most crashes. Remember that a crash can involve multiple vehicles.
Display the proportion of crashes in each month of 2019 where alcohol was determined to be the primary contributing cause.
Determine whether snowmobiles or recreational off-highway vehicles were involved in more crashes.
Display a cluster map showing the locations of crashes involving a hit and run.
crashes.csv: Each row in the dataset represents a traffic crash reported in Chicago.
crashes_vehicles.csv: Each row in the dataset represents a vehicle involved in a crash reported in Chicago.
Total Number of Reported Crashes: 474262
We start by making the contents of the pandas module available to our program. pandas is an easy to use open source data analysis and manipulation tool, built on top of the Python programming language. We will use it extensively throughout this project.
import pandas as pd
We import the contents of the crashes.csv file by calling the read_csv() method and store it in a DataFrame, named df_crashes. A DataFrame is a two-dimensional data structure with labeled axes, consisting of data, rows and columns. Think of it like a table built in Microsoft Excel or Microsoft Access. We then repeat the same action for the crashes_vehicles.csv file, naming it df_vehicles. For the crashes_vehicles.csv file, we drop the ‘CRASH_DATE’ column as it is also included in the other file.
df_crashes = pd.read_csv('crashes.csv', low_memory=False)df_vehicles = pd.read_csv('crashes_vehicles.csv', low_memory=False).drop('CRASH_DATE', axis=1)
To combine the two DataFrames into a single DataFrame, we call the merge() method of pandas. We pass three parameters: the first two are the names of the two DataFrames that we are merging and the third specifies the column common to both. Finally, with a call to the reset_index() function we reset the DataFrame index.
df = pd.merge(df_crashes, df_vehicles, on='CRASH_RECORD_ID').reset_index()
We use the print() function to print the string ‘Total Number of Reported Crashes:’ followed by the number of crashes in the DataFrame. The parameter that we pass to the print() function is made up of two parts. The first is the string ‘Total Number of Reported Crashes:’ surrounded in single quotes, denoting it as a string. The second part of the parameter is calculating the number of crashes in df. We use the nunique() method to calculate the number of unique CRASH_RECORD_ID in df, since a value can repeat.
print('Total Number of Reported Crashes:', df['CRASH_RECORD_ID'].nunique())
To convert the CRASH_DATE columns to a date type we use the pandas function to_datetime(), passing the columns to change as a parameter.
df['CRASH_DATE'] = pd.to_datetime(df['CRASH_DATE'])df_crashes['CRASH_DATE'] = pd.to_datetime(df_crashes['CRASH_DATE'])
We need to subset df and df_crashes to filter for crashes that occurred in 2017, 2018 and 2019. We use >= to represent greater than or equal to and <= to represent less than or equal to. Dates are in the format YYYY-MM-DD.
df = df[(df['CRASH_DATE'] >= '2018-01-01') & (df['CRASH_DATE'] <= '2020-12-31')]df_crashes = df_crashes[(df_crashes['CRASH_DATE'] >= '2018-01-01') & (df_crashes['CRASH_DATE'] <= '2020-12-31')]
We make the contents of the matplotlib and seaborn libraries available to our program. matplotlib and seaborn work together for creating visualizations in Python. I have a stylistic preference for my plots and set the seaborn theme style to darkgrid by calling the set_theme() method of seaborn. We also make the numpy library available to assist with labeling the axes of the plot.
import matplotlib.pyplot as pltimport seaborn as snssns.set_theme(style='darkgrid')import numpy as np
To make plotting easier and more visually appealing, we create a new column in the DataFrame to represent the hour within which each crash occurred. 0 represents 12am to 1am, 1 represents 1am to 2am, all the way up to 23 which represents 11pm to 12am. To achieve this we use .dt.hour on the CRASH_DATE column to obtain the hour and place it into a new column, Hour.
df['Hour'] = df['CRASH_DATE'].dt.hour
Now it is time for us to plot . We specify 15 x 8 as the size of the plot and then call the barplot() method of seaborn. We pass it 5 parameters, each of which we will take in turn. The first parameter is the data we wish to plot: we group the records by our new Hour column and then count the number of unique CRASH_RECORD_ID in each group using the nunique() method. The second and third parameters are the x and y for the plot, which are the Hour and count of crashes respectively. The fourth parameter is the color palette chosen and the fifth specifies no line around the bars, a stylistic preference.
The remaining lines of code set the title and axes labels for the plot.
plt.figure(figsize=(15,8))s = sns.barplot(data=df.groupby('Hour')['CRASH_RECORD_ID'].nunique().reset_index(), x='Hour', y='CRASH_RECORD_ID', palette='GnBu_r', linewidth=0)s.set_title('Hourly Number of Reported Crashes in Chicago (2018 - 2020)', y=1.02, fontsize=14)s.set_xlabel('Hour of Day', fontsize=13, labelpad=15)s.set_ylabel('Number of Crashes', fontsize=13, labelpad=15)plt.show()
CHEVROLET    49779Name: MAKE, dtype: int64
The first task we need to perform is to create a DataFrame containing only crashes that occurred during daylight lighting conditions. We start with df rather than df_crashes because we want to consider all vehicles involved in a crash. We do that by simply subsetting df for records with DAYLIGHT in the LIGHTING_CONDITION column and store the result in df_daylight.
df_daylight = df[df['LIGHTING_CONDITION'] == 'DAYLIGHT']
Now that we have our source data ready, we call the value_counts() method on the MAKE column in the DataFrame to calculate the number of occurrences of each MAKE. Because we only want to know the make of vehicle with the most crashes, we use nlargest(1) to display just the data what we need.
df_daylight['MAKE'].value_counts().nlargest(1)
We are able to use a single line of code for this step. Because it is a long line code, I will break it down in to parts for explanation.
We start by grouping the crash records in df_crashes by FIRST_CRASH_TYPE.
df_crashes.groupby('FIRST_CRASH_TYPE')['WEATHER_CONDITION']
Next, we want to determine which weather condition has the most crashes for each of the crash types. To do this we need to apply a function along an axis of the grouped data. Because we do not specify the axis, it defaults to 0, which is the index.
The lambda function calls the value_counts() method on each group (crash type) and counts the number of instances of each weather condition per group, in descending order. The call to the head() method then gives us the top weather condition for each crash type group.
.apply(lambda x: x.value_counts().head(1))
We use the the reset_index() method to reset the DataFrame index to the original one. We use it here to present the results in a more elegant manner. We pass the name parameter of ‘COUNT’ to name the column containing the count values.
.reset_index(name='COUNT')
Finally, we call the rename() method to rename the level_1 column as WEATHER, to make the output more readable.
.rename(columns={'level_1': 'WEATHER'})
Much of the code we use here is similar to the plot we created in step 3.
We specify 15 x 15 as the size of the plot and then call the countplot() method of seaborn. We pass it 3 parameters. The first is our df_crashes DataFrame. The second specifies that we wish to count the number of crashes for each primary contributory cause and by specifying y the bars will display horizontally on the plot. Because seaborn does not have an easy way to order the bars by size, for the third parameter order we tell seaborn to use the result of df_crashes[‘PRIM_CONTRIBUTORY_CAUSE’].value_counts().index to order the bars. This gives the descending order that we are looking for.
The remaining lines of code set the title and axes labels for the plot.
plt.figure(figsize=(15, 15))sns.countplot(data=df_crashes,  y='PRIM_CONTRIBUTORY_CAUSE', order = df_crashes['PRIM_CONTRIBUTORY_CAUSE'].value_counts().index)plt.title('Primary Contributing Cause of Reported Crashes in Chicago (2018 - 2020) ', y=1.01, fontsize=14)plt.xlabel('Number of Crashes', fontsize=13, labelpad=15)plt.ylabel('Primary Contributing Cause', fontsize=13, labelpad=15)plt.show();
We start with df rather than df_crashes because we want to consider all vehicles involved in a crash. We group the records by LIC_PLATE_STATE and then count the number of VEHICLE_ID for each group. We use this field, because it is a unique identifier for each vehicle in lieu of not having the actual license plate data. Because we want the top 10, we call the .nlargest(10) method. Finally we call reset_index() and name the count column as COUNT.
df.groupby('LIC_PLATE_STATE')['VEHICLE_ID'].count().nlargest(10).reset_index(name='COUNT')
Because we are only interested in 2019 crashes, we subset df_crashes using data filters to give us df_alcohol. With the second line of code we group the crashes by month and then count the number of crashes per month by calling the nunique() method, storing the result as df_total — this will be the denominator when we calculate the monthly proportions.
df_alcohol = df_crashes[(df_crashes['CRASH_DATE'] >= '2019-01-01') & (df_crashes['CRASH_DATE'] <= '2019-12-31')]df_total = df_alcohol.groupby(df_alcohol['CRASH_DATE'].dt.strftime('%m'))['CRASH_RECORD_ID'].nunique()
Now we need the numerator, which is the number of alcohol related crashes per month. Firstly, we group the crashes by the primary contributing cause, but only if they contain the words ALCOHOL or DRINKING. A call to the contains() method, passing the words as a parameter gives us what we need. We use the | character to separate the two words. Next we group the resulting records by month and then count the number of crashes per month by calling the nunique() method again, storing the result back in to df_alcohol.
df_alcohol = df_crashes[df_crashes['PRIM_CONTRIBUTORY_CAUSE'].str.contains('ALCOHOL|DRINKING')]df_alcohol = df_alcohol.groupby(df_alcohol['CRASH_DATE'].dt.strftime('%m'))['CRASH_RECORD_ID'].nunique()
Now that we have both the numerator and denominator, we can calculate the monthly proportions of crashes where alcohol was recorded as a primary contributing cause. We then call reset_index() like in other steps and use rename() to rename the columns for display purposes.
df_proportion = df_alcohol / df_total * 100df_proportion.reset_index().rename(columns={'CRASH_DATE': 'MONTH', 'CRASH_RECORD_ID': 'PROPORTION'})
Number of snowmobiles: 4
Number of recreational off-highway vehicles: 6
For this step we use the same code twice, firstly looking for snowmobiles and secondly for recreational off-highway vehicles. We subset df for each vehicle type and use the len() function to count the number of records in the respective subsets.
print('Number of snowmobiles:', str(len(df[df['VEHICLE_TYPE'] == 'SNOWMOBILE'])))print('Number of recreational off-highway vehicles:', str(len(df[df['VEHICLE_TYPE'] == 'RECREATIONAL OFF-HIGHWAY VEHICLE (ROV)'])))
We only wish to display the locations of crashes involving hit and runs, so we start by subsetting df_crashes to get the data we wish to plot, storing the result as df_hitrun. There are some records in the DataFrame without coordinates, so we remove them by using the notna() method.
df_hitrun = df_crashes[df_crashes['HIT_AND_RUN_I'] == 'Y']df_hitrun = df_hitrun[df_hitrun['LONGITUDE'].notna()]
To display our map we are going to use Folium and an associated marker cluster plugin. We start by making the contents of the Folium module available to our program and importing the plugins. Folium is very powerful and requires little code to create an interactive map with great detail.
import foliumfrom folium import plugins
We need to instantiate a Folium map, by passing the GPS coordinates of Chicago and a zoom level, which we will set to 12. The latitude we will use is 41.8781 and the longitude -87.6798 (I found these with a quick Google search).
m = folium.Map(location=[41.8781, -87.6798], zoom_start=12)
We are creating a cluster map, so use the MarkerCluster plugin, adding it to our map with a call to the add_to() method and passing our Folium map m as a parameter.
Then for each crash in df_hitrun we create a marker. We iterate through df_hitrun using a for loop. The zip() function creates a tuple of latitude and longitude for each crash, creating a marker for each. Finally, we display the map m. As you zoom in and out of the map, the clusters resize interactively.
hitrun = plugins.MarkerCluster().add_to(m)for lat, lng in zip(df_hitrun['LATITUDE'], df_hitrun['LONGITUDE']):    folium.Marker(        location=[lat, lng],           icon=None,    ).add_to(hitrun)m
I hope you have found this project a useful way to learn Python data analytics. Please share your feedback in the comments section.
Please also check out: Learn Python Data Analytics By Example: NYC Parking Violations and Learn Python Data Analytics By Example: Airline Arrival Delays.