The immense problem with air pollution (smog) that had developed by the early 1970s also caused a backlash. With schools being closed routinely in urban areas for "smog days" when the ozone levels became too unhealthy and the hills surrounding urban areas seldom visible even within a mile, Californians were ready for changes. Over the next three decades, California enacted some of the strictest anti-smog regulations in the United States and has been a leader in encouraging nonpolluting strategies for various industries, including automobiles. For example, carpool lanes normally allow only vehicles with two/three or more occupants (whether the base number is two or three depends on what freeway you are on), but electric cars can use the lanes with only a single occupant. As a result, smog is significantly reduced from its peak, although local Air Quality Management Districts still monitor the air and generally encourage people to avoid polluting activities on hot days when smog is expected to be at its worst.