Effects of roadway configurations on near-road air quality and the implications on roadway designs

- We investigate the impacts of elevated and depressed roadways, and roadside barriers.
- All configurations reduce ground-level air pollutant concentrations near roadways.
- The elevated roadway leads to reduction in both on-road and near-road concentration.
- Adding multiple features offers diminishing returns in concentration reduction.
- The effects of design features damp out <15 multiples of the characteristic height.

This paper presents an analysis of wind tunnel experiments of twelve different roadway configurations and modeling of these configurations using a Large-Eddy Simulation (LES) model, aiming at investigating how flow structures affect the impact of roadway features on near-road and on-road air quality. The presence of roadside barriers, elevated fill and depressed roadways, and combinations of these configurations all reduce ground-level air pollutant concentrations immediately downwind of roadways. However, all of these cases, except the elevated fill configuration, increase pollutant concentrations on the roadway itself. For a roadside barrier with finite length, higher concentrations than those without a barrier are present in a small region near the edge of the barrier, influenced by complex flow in that region which we term “Edge Effects”. The inclusion of multiple roadway features often result in lower downwind pollutant concentrations than those with single roadway features; however, adding features typically offers diminishing returns in concentration reduction. Generally, the effects on concentration, both beneficial and adverse will damp out within 15 multiples of the characteristic height, be it the barrier height or the elevation/depression height of the roadway. Thus, evaluating the trade-off between the air pollutant reductions near the ground and the air pollutant increases on the roadway and elevated above the ground will be important in designing a sustainable transportation system.