ABSTRACT
Over half of all humans live in cities, (UNPF 2007) with 1.75 billion more people expected by 2030 (McDonald et al 2011). Urbanization associated with the growth of cities transforms natural landscapes and impacts biodiversity, ecosystem processes and agriculture (Theobald et al 2000, Schwartz et al 2006, Grimm et al 2008a, 2008b, McKinney 2008, Satterthwaite et al 2010). Expanding transportation infrastructure is linked to the functionality of these expanding urban areas (Barthelemy et al 2013,
Schneider and Mertes 2014). However, transportation structures have adverse impacts to the natural environment (Trombulak and Frissell 2000, Forman et al 2003, National Research Council 2005), including: direct and cumulative mortality to species hit by vehicles (Seo et al 2013), reduced dispersal capacity (Forman and Alexander 1998), impediments to gene flow (Epps et al 2005), the spread of invasive species (Gelbard and Belnap 2003), the generation of greenhouse gas emissions (Fuglesvedt et al 2008, Kennedy et al 2009), and landscape fragmentation (Jaeger et al 2005, Girvetz et al 2008). Concern over the rapid growth of cities and transportation infrastructure highlights the need to reduce environmental impacts associated with this growth (Thorne et al 2005, 2006, 2009a) by initiating or improving requirements to offset those impacts via conservation or restoration of other lands, here termed environmental mitigation.
