During their annual breeding migration, red crabs will often have to cross several roads to get to their breeding grounds and then back to forest. As a result, red crabs are frequently crushed by vehicles and sometimes cause accidents due to their tough exoskeletons which are capable of puncturing tires. To ensure the safety of both populations, crabs and humans, local park rangers work to ensure that crabs can safely make their journey from the center of the island to the sea; along heavily traveled roads, they set up aluminum barriers whose purpose is to funnel the crabs towards small underpasses so that they can safely traverse the roads. Other infrastructure to assist the crab migration includes a five-metre-high "crab bridge". In recent years,[when?] the human inhabitants of Christmas Island have become more tolerant and respectful of the crabs during their annual migration and are now more cautious while driving, which helps to minimise crab casualties.