unit is usually accessed by opening a roll-up metal door, which is usually about the same size as a one-car garage door (smaller units may be accessed by a hinged metal door). A controlled access facility may employ security guards, security cameras, individual unit door alarms and some means of electronic gate access such as a keypad or proximity card. A few facilities even use biometric thumbprint or hand scanners to ensure that access is granted only to those that rent. Self-storage facility operators frequently provide 24-hour access, climate controlled storage, outdoor storage for RVs and boats, and lights or power outlets inside the storage unit as amenities to set themselves apart from competitors. Some storage facilities have open roofs i.e. a wire mesh roof which are not that secure, compared to ones that have full covered tin roofs that provide added security and privacy. In rural and suburban areas most facilities contain multiple single-story buildings with mostly drive-up units which have natural ventilation but are not climate-controlled. These buildings are referred to as "traditional" storage facilities. Climate-controlled interior units are becoming more popular in suburban areas. In urban areas many facilities have multi-story buildings using elevators or freight lifts to move the goods to the upper floors. These facilities are often climate-controlled since they are comprised mostly, if not totally, of interior units. Warehouses or grocery stores are sometimes converted into self-storage facilities. Loading docks are sometimes provided on the ground floor. Also, complimentary rolling carts or moving dollies are sometimes provided to help the customers carry items to their units. Urban self-storage facilities might contain only a few floors in a much larger building; there are successful self-storage businesses co-located with manufacturing plants, office tenants and even public schools. One in ten U.S. households now rent a self-storage unit. The growing demand for self-storage in the U.S. is created by people moving (some 40 million people move each year according to U.S. Census data), and by various lifestyle transitions, such as marriage, divorce, retirement, a death in the family, etc. Recent surveys of self-storage companies indicate a positive trend in market demand and occupancy rate. Over 54,000 self-storage facilities currently exist in the U.S. ranging from companies with a nationwide presence to companies with regional footprints or even stand-alone independent "mom and pop" facilities. Demand for storage space remains stable as of Q4 2015. The supply for self-storage