Early Research on Travel Demand
Interest in the relation between transit and the markets that support is as old as public transportation itself. Formal modeling of travel demand, the concrete conditions that influence individual decisions whether, where, and how to travel, only began with the large-scale transportation construction of the 1950's. Until quite recently, one of the greatest barriers to isolating the causes of what is known as "trip generation" has been reliance upon data of regional or city-wide resolution. Large-scale modeling techniques based on regional aggregates, however, were initially enough to suggest that effective transit and high-density land use were closely related.
A benchmark study of transit travel demand carried out by Boris Pushkarev and Jeffrey Zupan in the 1970's used aggregate density measures to determine thresholds for effective transit demand; these measures still serve as rules of thumb in transit planning today. Pushkarev and Zupan's study is the starting point for a brief review of the travel demand research leading up to the most recent, neighborhood-scaled studies of transit and location efficiency.
Pushkarev and Zupan began their study of travel demand with the observation that, today, transit functions in competition with the automobile. With the exception of neighborhoods within a handful of American cities, the percentage of trips carried by any given mode of transit or mode share is a small fraction of the total number of trips made. This has not always been the case. Before the expansion of the automobile market in the 1920's, and even into the early days of the 1950's suburban boom, transit was the most efficient way to travel distances longer than those easily traveled by foot. During the heyday of mechanized urban transit, from roughly 1880 to 1920, transit modes competed chiefly between themselves in a free market.
Because rail transport was so basic to economic activity at this time, it functioned as a spur to development. The functional design of the built environment was premised on the near and frequent operation of rail transport to serve the needs of inhabitants, merchants, and industry. This close relationship between rail transport and land use shaped the skeletons of the great American cities that came to maturity in the decades before the First World War.