2008). On the other hand, comparatively few studies (see overview in van der Ree et al. 2007) have addressed the extent to which the barrier effect of roads and road-related mortalities is reduced (Lehnert and Bissonette 1997; Dodd et al. 2004; Klar et al. 2009) or gene flow between populations has been enhanced by road mitigation measures (Corlatti et al. 2009; Clevenger and Sawaya 2010). Empirical studies that examine population-level effects of crossing structures

are even rarer (but see, e.g., Mansergh and Scotts 1989; van der Ree #learn more randurls[1|1|,|CHEM1|]# et al. 2009). Clearly, estimates of the extent to which a structure is used does not directly answer the question of to what extent the impacts of the road and traffic on wildlife have been mitigated. The paucity of studies directly examining the effectiveness of crossing structures on wildlife populations is exacerbated by the fact that such studies invariably permit, at best, weak inference. For example, many studies are of too short duration to distinguish transient from long-term effects. Only a small number of studies have employed a before-after design or included comparisons between treated and untreated sites (van der Ree et

al. 2007; Glista et al. 2009). Consequently, transportation agencies can rarely assess whether mitigation objectives have been met. Without well performed evaluations of the effectiveness of road mitigation measures, we may endanger the viability of wildlife populations and waste financial resources by installing structures that are not as effective as we think they are. Furthermore, we cannot establish a set MAPK inhibitor of best mitigation practices nor evaluate cost-benefits and consider what mitigation strategies are most efficient until effectiveness has been quantified. Here we propose a methodological framework for evaluating the effectiveness of wildlife

crossing structures. First, we identify the principle ecological objectives of crossing structures and discuss what needs to be measured to evaluate BCKDHB how well these objectives are being met. Second, we provide guidelines for study design, the selection of appropriate research sites, survey methods and the development of suitable/feasible sampling schemes. For cases where the mitigation is intended to benefit many species, we identify criteria to prioritise species for evaluation. Finally, we discuss the value of road mitigation evaluation for policy makers and transportation agencies and provide recommendations on how to incorporate evaluations into road planning practice. Guidelines for evaluating road mitigation effectiveness The first step in setting up a monitoring plan for evaluating the effectiveness of wildlife crossing structures (Fig. 1) is to determine the species targeted by the mitigation and to explicitly identify mitigation goals.