Why Does Red Mean Stop?

The l9th-century Scottish engineer Robert Stevenson, who was active in designing early lighthouses, (33)______ 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 (34)______ glasses available at the time, he found that red was a particularly intense light, meaning it (35)______ from the greatest distance.

So in maritime signaling red became an alternative to white, and was later adopted by the Admiralty in 1852 (36)______ the port-side on steam vessels. Green was adopted for the starboard-side, and vessels seeing the green light on (37)______ ships had the right of way.

When train tracks were developed, engineers adopted this system as meaning stop and go - and the same system continued with cars.