a section of Line 1 and 2. A few months later, on August 15, 1971 the line was extended 3 stations west, to . The line was extended again to in November. On April 23, 1973, the line was extended , at this point the line is long with 17 stations. On September 15, 1981 the line was again extended westward to creating a long line with 19 stations. The line was transferred to the newly created Beijing Mass Transit Operation Corporation and was officially opened to the public, ending the decade long trial operation period. By 1981, the annual passenger volume of Beijing Subway was 64.66 million passengers, with a daily average of 177,000 passenger trips. On August 15, 1986, the second phase of subway construction started and a feasibility study on the construction of the new line between of Fuxingmen to Bawangfen (Today's Beijing CBD) was carried out. A new branch heading east to a new station, , was completed between and stations on December 28, 1987. The section between and was transferred to the newly created Line 2 and Line 1 ran between Pingguoyuan and Fuxingmen stations. In January 1991, the feasibility study report on the construction of a new line between Fuxingmen to Bawangfen was approved with construction of the new "Fuba line", starting in June 1992. On December 12, 1992, Line 1 was extended east for one station to . In 1994, Line 1's signals were upgraded to automatic train protection system. On September 28, 1999, the first section of the Fuba line between to was opened. On June 28, 2000, the section between and stations opened, merging Line 1 and the Fuba Line to create a line with 23 stations. Headway Reduction Since 2014, the signaling and power systems has been gradually upgraded to allow for trains to run more frequently. The upgraded CBTC moving block signaling system in place today reduces rush hour headways from every 2 minutes and 5 seconds to 1 minute 45 seconds. Platform screen doors Due to platform congestion and suicides, a proposal was made in 2010 to refit all stations on Line 1 with platform screen doors. However, the ventilation system in the older stations of Line 1 was incompatible with full-height platform screen doors, so half height platform screen doors were chosen instead. In July 2016, the operator began installing platform screen doors on Line 1. In