The TRansportation and Air Quality (TRAQ) research group

Quantifying the effects of land-use and socio-economics on the generation of traffic emissions and individual exposure to air pollution at a metropolitan scale

The objective of this study is to better understand the generation of traffic-related air pollution at a metropolitan scale and identify the regions that are potentially the most affected by these emissions. We propose two measures of traffic emissions that potentially capture inequity in the spatial distribution of emissions: (1) the average level of emissions generated per individual and (2) the level of emissions occurring in a zone as a proxy for air pollution exposure. These indicators are estimated at the traffic analysis zone (TAZ) level by extending a regional traffic assignment model with capabilities for individual trip emission modeling while taking into account vehicle (type, age) and trip attributes (road type, speed, volume). We examine the spatial distribution of emissions as well as capture the determinants of emissions generated and exposed to through a multivariate regression analysis of the two indicators against a set of land-use and socio-economic variables. Our study area includes the Montreal metropolitan region, which covers an area of approximately 4,200 km2 and has a population of about 3.8 million.

The spatial distribution of results across the region is shown below. As expected, the high emitting individuals tend to reside on the periphery of the region, which is furthest from the central business district (CBD). Concurrently, the majority of low emitting individuals live centrally, on the island, much closer to the CBD. Overall, these results clearly confirm the intuitive hypothesis that high polluting individuals reside away from the downtown in suburban areas.

Exp_Map1 — The average emitted NOx per person (am and pm peak periods only) across the 1,552 TAZs in the region ranges from 0.0 to 17.5 grams.

Exp_Map2 — When we overlay the map of emissions occurring on the network onto the previous map, it is evident that most of the emissions occur in areas where the lowest polluting individuals reside.

Exp_Map3 — A cluster analysis divided the 1,552 zones into four clusters: 1) low emitter, high exposure; 2) low emitter, moderate exposure; (3) high emitter, moderate exposure; and (4) high emitter, low exposure. Based on the spatial distribution of the clusters, it is evident that the lowest emitting zones (highlighted in white and the lightest shade of grey) are also the ones that are exposed to the highest emissions. They are mostly located in central areas and in the CBD. In contrast, high emitting zones (dark grey) are also exposed to low amounts of pollution and located outside of the urban core.

Equity — We also compute a social disadvantage index (SDI) at the level of the traffic analysis zone. We wanted to highlight zones with high social disadvantage and high exposure levels. The SDI and normalized exposure were summed together and the spatial distribution of the resulting summation can be seen in the following figure. Zones with combined SDI and exposure rankings greater than 18 are of most interest, as these are the neighbourhoods that experience the ‘double-burden’ of greatest disadvantage and highest traffic-related air pollution levels. The majority of these zones border the major highways that surround the central part of the island of Montreal. However, they are not solely limited to the central areas as there are pockets of ‘high disadvantage + high exposure’ in the northeast of the island (Montreal Nord) and in Longueuil.