Land Use
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Urban areas that developed around public transportation are typically compact and densely developed: travelers walked to transit stations and made most of their daily trips by foot. As a result, housing, commerce and employment are spatially integrated, and clustered near or along transit lines. What we today call transit oriented development was originally development that was transit-oriented by necessity.
When the automobile became widely available and affordable to American families, travel speed within urban areas tripled, increasing the potential distance traveled within an hour at least nine times. Development in the age of the automobile could expand to one ninth of its original density without impacting travel time. Regions that experienced the bulk of their development after the auto revolution tend to have segregated land uses separated by larger distances: housing developments several miles from supermarkets and malls, and far from the nearest employment centers. Development around the automobile has resulted in a type of urban form that now makes other modes of transportation inconvenient and often uneconomical.
When urban planners speak of "land-use mix," they are referring to the balance of different land uses within a given area. A neighborhood with housing located above or beside shopping and entertainment, and near offices or industry, stands in contrast to the segregated land use that became predominant with the arrival of the automobile. Integrated land use, on the other hand, allows residents to conduct their daily tasks close to home, and multiple tasks in one trip, travel by alternative modes, and reduce automobile trips and trip lengths. Segregated land use is often accompanied by low transit accessibility and distances that make the automobile the only practical mode choice.
The importance of amenities close to home is emphasized by the fact that the majority of trips taken are for trips other than travel to work. In 1995, 76 percent of all household trips were to school, shopping, social and recreational activities, and other family business. If households are able to conduct the majority of those trips within a mile from home, or near a transit station, auto use diminishes significantly.
The type of land use affects the density of a neighborhood, which has a direct impact on the sustainability of transit in that area. Density is a measure of the concentration of human residence or other activity in a unit of land area. Types of density most relevant to travel demand are population—or residential density—and employment density. Residential density refers to the number of people per acre (or square mile) of land zoned for residential use. Employment density measures the number of jobs per unit area.
Public transportation is economically feasible only above certain minimum densities, while walking and biking are most practical for short trips in fairly dense areas. Households located in dense communities, such as those in the core of most older U.S. cities, travel shorter average distances to reach destinations. Walking, bicycling, or public transportation are more practical for these groups. Planning and zoning for the proper, often higher, densities is therefore a prerequisite for transit success.
Dense development has been shown to correlate with a reduced number of vehicle miles traveled and cars owned. The cost of owning and parking a car in dense areas makes other forms of transportation more attractive. Correspondingly, maps of emissions generated by vehicle travel in a county and per household within each county, show that households in dense, urban areas generate fewer auto trips than their rural counterparts. Employment density at trip origins and destinations has also been shown to relate closely to mode choice for both work and non-work trips, perhaps because it signals the availability of retail and service amenities.
Land use patterns are typically reinforced through municipal zoning codes. Though zoning alone is no substitute for comprehensive urban planning, it can encourage or discourage certain aspects of urban development supportive of transit. Parking requirements are a prime example. Zoning regulations specifying a certain minimum quantity of free parking per type of land use may in fact encourage single occupant vehicle use by hiding the true market costs of free parking. The presence of free parking at a place of employment served by Bay Area Rapid Transit was found in one case study to decrease the likelihood of using transit for the daily work commute by 20 percent.
Not all density is conducive to increased transit use, however. The most well-conceived transit oriented development will not effectively increase ridership if it is not part of a larger system that situates the origins and destinations of transit trips (such as home and work) in proximity to transit stops. Density that emphasizes one land use to the exclusion of others - commercial districts that empty out in the evening, or exclusively residential areas that offer no amenities or destinations, can discourage pedestrian activity and access to transit.
Many of the urban design principles of the Congress for the New Urbanism, such as public plazas, grid street design, a variety of pedestrian-scale design elements, and traffic calming measures, are found to have positive effects in conjunction with already sufficient densities. An internal study by Chicago's Metra commuter rail line examined four Chicago communities served by the line and concluded with an endorsement of pedestrian friendly urban design as a way of promoting ridership.
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