Spatial Analysis of Different Types of Traffic Accidents with QGIS
Purpose
The purpose of this tutorial is to give users an understanding of what free and open source software (FOSS) is and demonstrate how to use Quantum GIS (QGIS) by using spatial data. This tutorial will be analyzing the spatial data of different types of traffic accidents that occur in Barrhaven, Ottawa using QGIS.
Introduction
The city of Ottawa has many programs regarding the safety of transportation and safer roads. A Safer Roads Ottawa Program is committed to preventing or eliminating road deaths and serious injuries. The program is conducted by Ottawa Fire Services, Ottawa Paramedic Service, Ottawa Police Service, Ottawa Public Health, and the Public Works Department. They are conducting the program through culture change, community engagement, and developing a sustainable safe transportation environment. Another program called the Safety Improvement Program includes all three E's of road safety: education, enforcement, and engineering. Locations chosen to be studied are usually higher-than-average traffic collision rate locations. The Safety Improvement Program is carried out on an annual basis because that is when year-end traffic collision and volume data is available.
Data
The data used in this tutorial are available through the City of Ottawa's Open Data Website. The files used in this tutorial are a polygon shapefile of the wards of Ottawa, a point shapefile of traffic accidents, a polyline shapefile of Ottawa roads, a polyline shapefile of Ottawa railways, and a polyline shapefile of the pedestrian network. You may use any other data for another city or location as long as there is a polygon shapefile of your chosen city or neighborhood, a polyline shapefile for roads, and a point shapefile for accidents, as well as any other information you would like to analyze. To get ready for this tutorial, you should download all of these shapefiles and keep them in a working folder on your PC to make your work easier to locate.
Quantum GIS (QGIS)
Quantum GIS, also known as QGIS, is a great free and open source software (FOSS GIS tool). QGIS is great tool that enables users to create maps with the help of core functions and plugins that are available through the software, and at this site. QGIS is available for Windows, Mac OS X, and Linux. Download the long term release version (most stable) of QGIS (v.3.14) to begin this tutorial.
Tutorial
Now that you have downloaded the software and shapefiles, it is time to begin.
Setting Up the Environment
Go to the Project menu, select Projection Properties. The Projection Properties windows will open. Select NAD83(CSRS) / MTM Zone 9 as your projection due to the area of study being the Ottawa region. In previous versions one would have to toggle the Enable on the fly button, but in current versions of QGIS (3.0+), this is is on by default. Click Apply and OK.
Adding Shapefiles into QGIS
Once QGIS is launched, go to the Layer menu -> Add Layer -> Add Vector Layer. In the pop-up window, set your source type as File and click the Browse button under the Source header. Browse to where your shapefile is and double-click it or highlight it and select Open.
Insert each of the shapefiles you would like to analyze. Modify each shapefile to look the way you would like it to by double-cliking the layer from the Layers Panel on the left hand side, then selecting Style from the left hand side of the popup window. Here you can change the symbol type (such as gradient, single symbol, categorized etc), the colour, the size and more.
Editing Attribute Tables: Selecting by Expression
Right click on the accidents layer that you have imported and select Open Attribute Table. An attribute table of the accident layer will appear containing a multitude of different fields, including the different types of impacts and the street names where the accident occurred. Once you have the attribute table open, select Select Features Using an Expression (
). A Select By Expression window will appear, and in the function list, expand the Fields and Values and select IMPACT_TYP. Select the All unique button underneath the Search Bar. Doing so will bring up a list of the unique values from the chosen attribute. In this case, choosing IMPACT_TYP will list all the different impact types observed in the attribute table.
The expression box allows the user to create formulas that aid in simplifying the search process for the field desired. In the expression box, double clicking a field adds it to the expression. Double clicking on the IMPACT_TYP will cause "IMPACT_TYP" to appear in the expression window. Once that appears, you select the equal (=) sign where the available operators are listed. The next step is to select a field value. In this example, the first field value selected is Approaching. The expression listed in the expression window should read: "IMPACT_TYP" = 'Approaching'. Upon hitting Select Features in the bottom right corner, the impact types that are Approaching are now highlighted on your map.
Saving Selected Points (Shapefile)
The selected points that are now highlighted can be saved as their own shapefile. To do this, right click on the accidents layer, mouse over Export, then select Save Selected Features As and a window will appear.
In this window, the format that the shapefile should be saved under is "ESRI Shapefile". Additionally, you will be given the option to set the CRS of the shapefile you are about to create. Set the CRS to the Project CRS, in this case it will be Project CRS: ESPG:2951 - NAD83(CSRS)/MTM zone 9. Name the file what you please, and then click OK. Doing so will create and save the shapefile in the location/folder where the Map Project is saved.
Repeat both the Editing Attribute Tables: Selecting by Expression and Saving Selected Points (Shapefile) steps until all the impact types have been saved as their own individual shapefiles.
Selecting a Specific Area
To select a specific area, open the attribute table for the Wards shapefile layer. Click on the Select features using an expression () button.
The Select By Expression window will appear, and in the function list, expand the Fields and Values and select the specific area you want to study. In our case, Barrhaven is the chosen site. In order to select only Barrhaven, we can use a number of different options for the expression: "WARD_EN" = 'BARRHAVEN' , "WARD_NAME_" = 'Barrhaven' , "WARD_NAM_1" = 'Barrhaven' , "DESCRIPTIO" = 'BARRHAVEN - Jan Harder' . Any of them do the job.
If we were tasked with selecting more than one ward at a time, our expression would look something like: "WARD_EN" = 'BARRHAVEN' OR "WARD_EN" = 'CUMBERLAND' . Once the wards you want are selected, save and create shapefiles for the selected layers as described in the Saving Selected Points (Shapefile) section.
Clipping Files
When the newly merged shapefile has been made, you can simply clip all of the other information (accidents, roads, railways, pedestrian paths) onto the study area as well. To do this, go to the Vector Menu, select Geoprocessing Tools and then select Clip.
A clip window will appear and the input vector layer will be what shapefile you want to clip onto the output vector layer. For example, in order to clip all the Side Swipe Accidents that occurred in the Barrhaven area, the input vector layer should be Side Swipe Accidents and the overlay layer would be the Barrhaven shapefile that was created in the previous selection. For output shapefile, you can browse to the folder that contains all of your other shapefiles and choose a name for your clipped file.
When clipping the other shapefiles, the overlay layer remains the same, but the input vector layer changes. Proceed to clip all the other vector layers as done above, and format them to the style of your liking. Once completed, you should have a map similar to something below.
Analyzing the Data
In order to determine how many of each accident type has occurred, one can right click the layer in the Layers menu on the bottom left of the display, and can "check" the Show Features Count, which then displays the number of occurrences for the layer beside the layer name.
Lets say that we wanted to determine how many Rear End accidents occurred near Local roads, we would first create a buffer around the clipped rear end accidents shapefile. To do so you select Vector -> Geoprocessing Tools -> Buffer. In this example, a buffer distance of 25m was used and the segments to approximate value was 25. The created file will be temporary, which we can then later save if desired.
Once we have the desired buffer for the desired accident type, we can then filter and use the Select by Expression option in the attribute table of the clipped Roads shapefile to filter and select the roads whose "SUBCLASS" = 'LOCAL' . Once this is completed, the local roads will be highlighted in yellow on your screen.
With these two steps completed, we then use the intersection tool found by selecting Vector -> Geoprocessing Tools -> Intersection. Our input layer will be the clipped Road shapefile with the "Selected features only" button checked, and the overlay layer being our temporary Buffered shapefile that we created in the first step of this section.
This will again create a temporary shapefile. Once created we can again determine the count of the new shapefile, which in turn tells us how many Rear end accidents occurred within 25 meters of a local road. In this case, 36.
Using these methods, you can perform a multitude of operations to interpret things such as how many accidents occur within a distance from pedestrian pathways, how many accidents occur within a ward, how many accidents occur within a distance from railways, how many of a certain accident type occur within a distance from other accident types etc.
Making a Map
Once we have produced the data that we want to showcase, we can use QGIS to make an aesthetically pleasing map. First, turn on all of the layers that you would like to be showcased and turn off all others by selecting and unselecting the checkbox next to each layer name. I want to showcase different types of accidents that occur within Barrhaven so I have selected these layers:
Select Project -> New Printer Composer and then type a name for your map in the text box and select OK (it does not have to be the same as the title you place on your final map). In the pop-up window, select Layout -> Add Map (
) and then draw a rectangle on your canvas where you would like your map. To reposition your map display, select Layout -> Move Content. Now you can drag your map into place and zoom in and out. Under the Item Properties tab on the right hand side, you can also change the scale to get the perfect sizing.
To add a title, select Layout-> Add Label and draw a box for your title. Under Item Properties -> Label you can type your title in the textbox and change the font type, size, colour etc.
To add a north arrow, select Layout -> Add Image and draw a box on your canvas. Under Item Properties -> Search directories you can view different North Arrows and choose which one you would like to use.
Similarly, you can add a scale bar by selecting Layout -> Add Scalebar.
To add a legend select Layout -> Add Legend. In the Item Properties tab, there is a Legend Items menu. Here you can adjust which layers you wish to add to your legend. Should you have multiple layers that you don't wish to add, you can uncheck the "Auto Update" box, which will then allow you to freely remove legend entries. To rename legend items, double click a legend item and enter the text you want displayed. You can also rearrange legend items by dragging them up and down, or using the up and down arrow
(
) buttons underneath the list of legend entries.
Once you are happy with your map, you can export it as an image, SVG or PDF by selecting Composer from the top left hand menu and choosing either Export as Image, Export as SVG or Export as PDF. Here is an example of a completed map of Barrhaven that indicates locations of different types of traffic accidents from 2018.
Conclusion
QGIS is an intuitive and user-friendly software that in this instance has allowed the production of a map that showcases traffic accidents in the Barrhaven ward of Ottawa as well as allowed the user to analyze different scenarios. This is just an example of the many ways that a few shapefiles can be combined to produce a multitude of useful and easily understood data
References
- City of Ottawa. (2020). OpenData Ottawa | City of Ottawa. Retrieved October 6, 2020, From City of Ottawa: http://data.ottawa.ca/
- City of Ottawa. (2014). Safer Roads Ottawa Program | City of Ottawa. Retrieved December 20, 2014, from City of Ottawa: http://ottawa.ca/en/residents/transportation-and-parking/safer-roads-ottawa-program
- City of Ottawa. (2014). Safety Improvement Program | City of Ottawa. Retrieved December 20, 2014, from City of Ottawa: http://ottawa.ca/en/residents/transportation-and-parking/road-safety/safety-improvement-program
- MacOdrum Library. (2014). Maps, Data and Government Information Centre | MacOdrum Library. Retrieved December 19, 2014, from MacOdrum Library: https://www.library.carleton.ca/contact/service-points/maps-data-and-government-information-centre
- QGIS. (2014). Discover QGIS. Retrieved December 20, 2014, from QGIS: http://www.qgis.org/en/site/about/index.html
- QGIS. (2014). Download QGIS. Retrieved December 20, 2014, from QGIS: http://www.qgis.org/en/site/forusers/download.html
