The first traffic signals showing red and green lights were installed outside which London building?

The first traffic signals showing red and green lights were installed outside the Houses of Parliament in London.

The manually operated signals were installed on 9 December 1868 near Parliament Square. They were intended to help pedestrians—especially Members of Parliament crossing Bridge Street—navigate heavy traffic safely. The device used red and green lights at night, adapting the established color language of railway signaling.

The proposal came from J. P. Knight, a railway engineer from Nottingham. Railway signaling engineers Saxby & Farmer constructed the apparatus, which was operated by a police officer. Red indicated stop and green indicated caution or permission to proceed, although the system was much less sophisticated than modern electrically controlled signals.

The experiment ended after about a month when a gas leak caused an explosion, injuring the operator. London would later develop more durable traffic-control technology, and electrically operated signals became standard in the twentieth century. The 1868 installation is nevertheless widely treated as the beginning of modern traffic lights.