Jennifer Dill and Theresa Carr's research on bicycle transportation in 35 U.S. cities also suggests that "higher levels of bicycle infrastructure are positively and significantly correlated with higher rates of bicycle commuting"; and a 2010 study comparing streets in Copenhagen that had had cycle tracks and bicycle lanes added to them found that cycling volume increased 20%. However, on the cycle track streets bicycle accidents increased 10% more than would be expected from the changed bicycle and automobile traffic volumes, making the cycle tracks less safe for cyclists than the unmodified roads. Streets with bicycle lanes added saw a 5% increase in bicycle traffic but a 49% increase in bicycle accidents. Despite this, the study notes that "the gains in health from increased physical activity increased numbers cycling are much, much greater than the losses in health resulting from a slight decline in road safety." Although prior studies did not differentiate between new bike riders and those that had changed routes due to the new facility, recent research has shown that the greater level of protection, the greater number of new bike riders (as opposed to riders who used the route before construction or shifted routes).