Orange County has a population of about 3 million. It is primarily located on a coastal plain and low foothills southeast of the city and county of Los Angeles, CA. Orange County is nearly fully developed. Some large vacant lands remain in the most southerly areas of the county. Two large decommissioned military bases are being redeveloped in the cities of Irvine and Tustin, CA, and many communities are experiencing significant growth through infill and redevelopment activities. All tolls along the 91 Express are paid by transponder. There are no cash toll collection facilities. Visitors should not plan to drive along the lanes without a FasTrak transponder. As a result, the transponder possession rate is extremely high for residents along the 91 corridor. The 91 corridor remains a major circulation issue for Orange County and communities to the east. An ongoing major investment study is analyzing alternatives for improving mobility in the corridor.