Changes of street use and on-road air quality before and after complete street retrofit: An exploratory case study in Santa Monica, California

The California Complete Streets Act of 2008 requires local governments to update general plans so that new construction or modification of roadways considers all transportation modes, which include but are not limited to walking, cycling, and driving. This work evaluates the effect of a complete street retrofit on Ocean Park Boulevard (hereafter referred to as "the retrofit") in Santa Monica, California, in terms of the street use by different transportation modes and corresponding ultrafine particle (UFP) and fine particle (PM_2.5) concentrations. After subtracting background concentrations, UFP decreased after the retrofit by 4200particlescm^-3 while PM_2.5 had no statistically significant change. The emission-weighted traffic volume, an index used to account for flows of vehicles with different pollutant-emitting capacities, decreased 26%, which may explain why UFP reductions were observed while total traffic flow remained the same. The number of pedestrians increased by 37% compared to pre-retrofit conditions and the number of cyclists remained approximately the same. Although no causality could be reached, this study observed improved air quality on street after retrofit. Nonetheless, a full evaluation of the health impacts of the retrofit requires further information about how travel behavior, not just traffic, has changed.