Why Does Red Mean Stop?

The 19th-century Scottish engineer Robert Stevenson, who was active in designing early lighthouses, (7) ______ for an alternative colour to white — most lighthouses had a white beacon — when he built a lighthouse near to one that already existed, because he was afraid ships wouldn’t  be able to tell which was which. Of the light sources and (8) ______ glasses available at the time, he found that red was a particularly intense light, meaning it (9) ______ from the greatest distance. So in maritime signalling red became an alternative to white, and was later adopted by the Admiralty in 1852  to mark the port-side on steam vessels. Green was adopted for the starboard-side, and vessels seeing the green light on other ships had the right of way.

When train tracks were developed, engineers adopted this system as (10) ______ stop and go — and the same system continued with cars.

The 19th-century Scottish engineer Robert Stevenson, who was active in designing early lighthouses, (7) ______ for an alternative colour to white ...