and the appearance of the street as well as the public safety has been greatly enhanced." By 1917, the number of incandescent filament lamps used in street lighting had reached 1,389,000 across the United States, while the number of arc lamps had started to decline. Mercury vapor lamps By the mid-20th century, increasing motorization necessitated better illumination, particularly in business districts where there was more mixing of cars and pedestrians, as well as along commercial thoroughfares. Streets needed to be illuminated more evenly, and a minimum level of lighting needed to be maintained throughout the night. Street lighting became a major expense for US cities, which sought to control costs in various ways. From the point of view of a municipal lighting department, the fact that incandescent lamps "radiate with very low efficiency, producing relatively little light at visible wavelengths", made them less and less attractive for public street lighting. Mercury vapor streetlights started to be used more widely in the United States after 1950, mainly due to their cost efficiency. By then, the lifespan of mercury vapor lamps had been extended to 16,000 hours, and they could provide up to 40 lumens per watt, whereas incandescent lamps could only deliver 16 to 21 lumens. The first large street installation of mercury vapor lamps in the United States was in Denver, Colorado, on Park Avenue, where they were used together with incandescents. In 1964, nearly 39 percent of street lights in the US were mercury vapor, while incandescents accounted for 60 percent. By 1973, the use of incandescent outdoor lamps was rapidly declining, while the production of mercury vapor lamps soared. Sodium vapor lamps Low-pressure sodium vapor street lights produce a strong monochromatic yellow light, which also reveals more detail to the human eye, even at low levels of luminance. In the 1930s, sodium vapor lamps were not efficient enough to make them a compelling alternative to incandescents. However, because they enhanced visual acuity, they were recommended for safety lighting in tunnels, on bridges, and at "cloverleaf" interchanges on express highways. In the US, street lights using sodium vapor were first installed on a rural highway near Port Jervis, New York, in 1933. In 1938, a study of sodium vapor light use at selected intersections in Chicago claimed that the new lighting had helped to reduce the number of accidents in those areas. Lamp manufacturers started to promote sodium vapor