# Design of Road-Side Barriers to Mitigate Air Pollution near Roads
## 3 Results and Discussion
### Including Cost in the Selection of Barrier Size and Geometry  
Figure 10 shows the dispersing performance of the barrier geometries considered in this study when they are located at the road edge (\(d^{\star}=0\)) as a function of the barrier size (\(h^{\star}\)). We highlight that in this figure, the air pollutant dispersing performance of the barrier is expressed as the fraction of the downwind ground concentrations (C*/C*_rf_). Therefore, the best performance is observed when this ratio tends to zero.  
Figure 10 also includes the relative cost (\(S_{r}\)) calculated as described in Section 2.6. This figure also shows that the rate of gain in the dispersing performance with respect to the barrier height (\(d\)(C*/C*_rf_)/_dh*_) tends to decrease with \(h^{\star}\). However, the rate of cost increment (\(dS_{r}\)/_dh*_) remains constant. Therefore, for a given barrier geometry, the intersection of the C*/C*_rf_ curve with its corresponding \(S_{r}\) line qualitatively represents the barrier size with the most favorable cost-benefit ratio. Larger barriers require greater economic resources per unit of dispersing performance gained.  
According to the above reasoning, the best cost-effective dispersing barrier has a geometry of a quarter ellipse oriented toward the road with a height of \(h^{\star}\)-0.155 (\(h\)-1.62 m for a road 10.5 m wide) and located at the road edge. This barrier reduces 76% of the concentration observed near the road without any barrier. It is important to note that this alternative achieves higher pollutant reduction percentages than what has been typically reported for vertical barriers in the presence of gas phase pollutants [11; 15; 21].  
Similar dispersing capacities can be obtained with the vertical barrier. However, it requires a larger barrier (\(h^{\star}=0.29\) or \(h\)-3.05 m for a road 10.5 m wide) with a slightly higher relative cost (\(S_{r}=2.8\)) than for the case of the quarter ellipse barrier (\(S_{r}=2.4\)).  
The above analyses are general recommendations in the design of barriers for mitigating the pollution near roads. However, the final design of the barriers needs to include additional considerations such as local aspects (actual barrier construction cost) and other specific needs (minimum and maximum distances to the road, feasibility for vehicles to get close to the barrier, etc.).  
Results reported in this work can be used in multiple applications such as traffic-related emissions, industrial gas pipe leaks, and in general, any area or line-source of pollutants emitted on flat terrain. Results may be used to quantify the exposure level to  
Figure 10: Dispersing performance measured as (C*/C*_rf_) and relative cost (\(S_{r}\)) of different barriers located at the road edge (\(d^{\star}=0\)) as a function of the barrier size (\(h^{\star}\)). Intersections indicate the barrier size with the most favorable cost-benefit ratio for each barrier geometry considered.  
air pollutants of the population living near highway-type roads, as well as to identify effective countermeasures to mitigate the impact of said pollutants. It is important to note that the study of synergic effects of additional barrier properties such as vegetative covers, structural holes, and the use of pollutant-absorbent or catalytic paints are outside the scope of this work. Additional work is required to confirm that results obtained in this study, when expressed in dimensionless numbers, are independent of the road width. Barrier edge effects in the crosswind direction were neither considered in this study.