1 Department of Civil and Systems Engineering, Johns Hopkins University, Baltimore, MD 21218, USA.
2 School of Public Health and Health Sciences, University of Massachusetts Amherst, Amherst, MA, USA.
3 Department of Epidemiology, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, USA.
4 Department of Civil and Systems Engineering, Johns Hopkins University, Baltimore, MD 21218, USA; Department of Epidemiology, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, USA. Electronic address: l.gardner@jhu.edu.