At the beginning of the 20th century, many cities in the United States, the United Kingdom, and elsewhere had extensive streetcar/tram or interurban networks that played important roles in local passenger and cargo transportation. However, over the next half-century or so, in parallel with the development of the automobile and the bus, streetcar networks tended to decline, so that by the early 1960s very few systems remained in operation in most countries. For example, my own hometown, Houston, despite its reputation as a car-oriented city on par with Los Angeles, had a network of nearly 100 miles of trackage around 1920, servicing most of the city, only for it to gradually be replaced by buses and be shut down entirely in 1940. Ironically, only a few decades after most of these systems shut down, rising gasoline prices and a new environmental awareness caused interest in streetcar-like systems, now termed "light rail," to increase from 1970 onwards, so that many cities today are studying or building new light rail and streetcar systems.
The question, then, is how this decline could have been minimized, so that more streetcar systems manage to survive to the present day, and what effects would more surviving systems have, generally speaking?