Most traffic engineers consider the street hierarchy to be optimal, since it eliminates through traffic on all streets except arterials. However, some have contended that it actually exacerbates traffic congestion, leading to air pollution and other undesirable outcomes. An alternative to street hierarchy, Traditional Neighborhood Development (TND) networks, recommended by the Institute of Traffic Engineers, implies that a type of hierarchy is desirable nonetheless. It suggests that "While TND street networks do not follow the same rigid functional classification of conventional neighborhoods with local, collector, arterial and other streets, TND streets are hierarchical to facilitate necessary movements."