The definition of "the border" has expanded from a physical line or wall separating two nations, to an amorphous binational region. Day visitors from Mexico may legally travel without a visa in an area extending to 25 miles (40 kilometers) from the international border, as defined by the U.S. Department of Homeland Security.4 Along the San Diego-Tijuana border, the border commercial zone, within which commercial vehicles from Mexico can travel, has been extended to 45 miles or to the city limits of Oceanside. The 1982 U.S.-Mexico Border Environmental Agreement defined the border as a region encompassing 100 kilometers (62 miles) on either side of the U.S.-Mexico border.