These "modern" specific mercury vapor fixtures were introduced by Westinghouse in 1947 and employed the first horizontally-operated 400 watt mercury lamps (those also were a Westinghouse "first" in the industry, also.. the same year). The latter had quartz arc tubes inside of them so that these bulbs could be operated in any position. It took GE a couple years to develop this innovation.

Postwar downtown urban and suburban development was fast-paced and street lighting upgrading throughout the US was everywhere. The forementioned mercury vapor lamps were twice as bright as their incandescent counterparts and lasted several times longer.

FYI..Older mercury lamps of the 1940s had to be mounted vertically, did not last as long and were not as energy-efficient as then-newly introduced Westinghouse ones as described here.