Mapping of Motor Vehicle Injuries — NYC
By: Mateos Shehu
Tags: GIS, Car Accidents, NYC

3 — ROAD ANALYSIS

A spatial analysis of the road network identifies what is the amount of accidents resulting in injuries in each road. Since a road shares the same physical characteristics across all its length, the assumption is that the accidents in any part of a specific road also should share the same characteristics between them. Examples might be roads with with heavy traffic where driver inattention may predominate the type of accidents or roads with a high sped limit where the speed can result in accidents.

The analysis is carried out with consideration that some roads are longer so the number of point features (accidents) which falls in them is higher. The Road network, like the neighbor network, is normalized by proposing a coefficient equal to the number of accidents in each road divided by the length of the road. Road network is also important to study since roads can go through multiple boroughs, thus its a feature that neighborhood analysis cannot capture. The vector layer for the road is downloaded from the NYC Open Data Portal “NYC Street Centerline”

Results

The analysis show that some of the most dangerous streets are located in the Bronx like the Bruckner Expressway and the Jerome Avenue. For Manhattan the most dangerous roads are the 1st, 2nd, 3rd avenues and Broadway. In the Queens the most dangerous roads are the Queens and Woodhaven Boulevards. Brooklyn has Fulton Street as its most dangerous and several others equally dangerous. Staten island from the Road analysis is the safest Borough where all of its roads are below average regarding number of accidents resulting in injuries.

Steps taken in QGIS

“Vector” > “Geoprocessing Tools” > “Dissolve”. This will merge together all the roads which share the same unique ID.

“Attribute Table” > “Field Calculator” > “Function” > “$ID”. This will create a unique ID field, which we will later use to join. Since spatial operation can only be done on polygon layers, we would need a unique ID to perform a table join between the road line and the road polygon which will have the analysis data

“Vector” > “Geoprocessing Tools” > “Buffer”. This command will create a buffer zone of the line feature. It is important to create a polygon feature from the roads in order to perform the spatial analysis of counting polygons. QGIS has no direct way of calculating the number of points near a line like ArcMAP does, so to overcome this difficulty the calculation is done within a polygon feature and a point feature. The buffer is with a distance of 80feet

“Vector” > “Analysis Tool” > “Count points inside Polygon”. This will count the points (car accidents) which fall inside the roads and return the total number of accidents in a unique road. Then the roadpolygon.dbf file is saved as csv using excel.

“Layer” > “Add delimited text layer”. This brings the roadpolygon.csv in the QGIS file we are using

“Properties” > “Join” we take all the attributes from the polygon to the line by using the unique id previously created.

“Properties” > “Symbology” > “Graduated” > “Method — Size”. This creates the graduate map we see above. The choice of using size rather than color, to graduate the map is because the difference is size is more visible than difference between colors