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High Density Traffic Considerations

An important question when considering rail and rapid bus transit is whether the community has the appropriate population density and land use patterns to support such systems. Much of the popular rail and mass transit print media touts the success of rail and rapid bus transit but glosses over the issues of density, accessibility and the serious land use and transportation planning efforts communities must make in order to have successful systems. The resource “A Toolbox for Alleviating Traffic Congestion,” published by the Institute of Transportation Engineers (ITE), supplies some general observations to consider when assessing transit options. It concludes that transit success is improved by higher densities at the origin (defined as home) or the destination (work, shopping, etc.) (ITE 90). But the mix and types of activities and land use are also important: for example, 9-to-5 office workers have different transit needs than a rotating hospital staff (ITE 90). Charlottesville and Albemarle County have this sort of wide variety in transit demand because the dominant employer, the University and its accompanying hospital system, are juxtaposed with a growing number of retail stores and small information service companies located downtown and in suburban research/office parks.

The Transit Cooperative Research Program (TCRP) doesn’t work with such absolutes in its transit literature review, though the bottom line on density is similar to that of ITE. In its review of empirical literature, TCRP cautiously offers “working hypotheses” for guidance (Miller et al. 33). It labels specific researchers as “pro density” because they consider density to be the single most important factor in explaining transportation related differences between cities (Miller et al. 33). But TCRP’s review combines the density issue with factors such as the variety of transportation choices, socio-economic features of commuters, and the availability of cars.

To simplify the density issue and give some substantive guidance to the transit planning process, the ITE resource offers a possible baseline. ITE suggests several minimum density levels for correspondingly intense transit investments. The minimum level is local bus service (one bus per hour), which should serve residential areas averaging 4 to 5 dwelling units per acre (DUA) or a gross population density of 3,000 to 4,000 people per square mile. At this level, a 5 to 8 million square foot concentration of non-residential floorspace is sufficient support for buses (ITE 90). The next level of service is intermediate service (one bus every half hour), which can serve 7 dwelling units per acre or a gross density of 5,000 to 6,000 people per square mile. Nonresidential concentrations needed for this stage of service range from 8 to 20 million square feet (ITE 91). Frequent bus service (a bus every 10 minutes), typically serves 15 dwellings per acre or 10,000 people per square mile. A range of 20 to 50 million square feet of non-residential focused activity is suggested at this service level (ITE 91).

Once a city reaches the frequent level of service, the ITE recommends that express buses, or busways, be explored. At the frequent level, the speed of bus service declines as street congestion and the number of passenger pick-up/drop-off points rises (ITE 91). Bus rapid transit, with exclusive rights-of-way, provides a potentially costeffective alternative to other transit modes. This approach is explored later in this review.

One of the alternatives to buses, once residential densities reach 9 dwellings per acre in the bus line’s catchment area (between ¼ and ½ mile of route) and a nonresidential concentration of 35 to 50 million square feet, is light rail transit (ITE 91). If rights-of way can be found at grade, the requisite concentration can be lowered to 20 million non-residential square feet due to lower capital costs. Commuter rail service can accommodate densities as low as 1 to 2 dwelling units per acre (ITE 90). But these commuter rail corridors, with the accumulated volume necessary, lead to non-residential groupings of 100 million square feet or more, which are found only in the nation’s largest cities.

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