<P> Since the September 11 attacks, the roads and grounds around the Capitol have undergone dramatic changes . The United States Capitol Police have also installed checkpoints to inspect vehicles at specific locations around Capitol Hill, and have closed a section of one street indefinitely . The level of screening employed varies . On the main east - west thoroughfares of Constitution and Independence Avenues, barricades are implanted in the roads that can be raised in the event of an emergency . Trucks larger than pickups are interdicted by the Capitol Police and are instructed to use other routes . On the checkpoints at the shorter cross streets, the barriers are typically kept in a permanent "emergency" position, and only vehicles with special permits are allowed to pass . All Capitol visitors are screened by a magnetometer, and all items that visitors may bring inside the building are screened by an x-ray device . In both chambers, gas masks are located underneath the chairs in each chamber for members to use in case of emergency . Structures ranging from scores of Jersey barriers to hundreds of ornamental bollards have been erected to obstruct the path of any vehicles that might stray from the designated roadways . </P> <P> In 2004 the capitol was briefly evacuated after a plane carrying the then governor of Kentucky strayed into restricted airspace over the district . A shooting incident occurred in March 2016 . One female bystander was wounded by police but not seriously injured; a man pointing a gun was shot and arrested, in critical but stable condition . The city police of Washington D.C. described the shooting incident as "isolated". </P> <P> The United States Capitol Visitor Center (CVC), located below the East Front of the Capitol and its plaza, between the Capitol building and 1st Street East, opened on December 2, 2008 . The CVC provides a single security checkpoint for all visitors, including those with disabilities, and an expansion space for the US Congress . The complex contains 580,000 square feet (54,000 m) of space below ground on three floors, and offers visitors a food court, restrooms, and educational exhibits, including an 11 - foot scale model of the Capitol dome . It also features skylights affording views of the actual dome . Long in the planning stages, construction began in the fall of 2001, following the killing of two Capitol police officers in 1998 . The estimated final cost of constructing the CVC was $621 million . </P>

Which side of the capital is the senate