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time spent waiting for a transit vehicle to arrive is especially onerous and is a significant deterrent to ridership. In general, service headways longer than 15 minutes can be a major disincentive for users with other travel choices available. Even better than 15-minute headways for attracting transit riders are shorter service intervals that allow going to the transit stop without having to worry about the specific service schedule. Most authorities agree that 8- to 10-minute service headways or shorter are required to provide this degree of flexibility for riders. Although effects of transit service frequencies have been relatively well studied in various contexts, no satisfactory quantitative evaluations covering direct effects on TODs per se have been encountered. There is no reason to expect, however, that general findings concerning transit frequency effects on mode choice and ridership would not apply as well to TODs. Effects on TOD residency choice remain to be quantified. Transit accessibility is a measure that combines the metrics of service coverage, frequency, and travel time. It has been shown to be an important indicator of traveler response associated with station-area development. Accessibility reflects the number and importance of locations that can be reached by transit within a reasonable travel time. A multiple regression research model of project- level transit mode share, already presented in Table 17-28, found greater relative job accessibility by transit to be an important factor in explaining increased transit mode share among rail-based housing projects, all else being equal (Lund, Cervero, and Willson, 2004a). Highway Access and Congestion Highway access, even with high transit use, is important to TOD--especially in the suburban con- text. A substantial number of residents, employees, and customers will inevitably travel to and from a TOD using private vehicles. The 2003 California TOD travel characteristics study found 71 percent of resident work trips and 88 percent of non-work trips were made by automobile, averaged across the residential station areas surveyed (Lund, Cervero, and Willson, 2004a). Many TODs are built in locations that feature good highway access, sometimes simply because of where station-adjacent land is available, sometimes to meet adequate public facility requirements, and partly in the interests of having a more saleable development. However, when highway travel times are too much faster than transit travel times, more people choose to drive. For this reason, good highway access does not generally contribute to higher transit ridership figures except to the extent that it enables a larger development than would otherwise be possible. The higher densities associated with the typical TOD may contribute to localized congestion. (For more on this topic see the discussion under "Trip Characteristics and Congestion" within the "Related Information and Impacts" section.) To the extent that any such congestion causes automobile travel times to decline relative to transit services operating on an exclusive right-of-way or in reserved lanes, it will tend to encourage transit use at the TOD. Similarly, walking rather than driving may be encouraged for short trips to the extent that good pedestrian connections are available. One analysis--vividly illustrating the unknowns involved in determining causality--has suggested that the deterrent effects of roadway congestion and parking challenges created by the high-density development (in other words, 17-63
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automobile use inconveniences) are more responsible for shifts from automobile use to transit and walking in such settings than the efforts to make transit use and walking attractive (Chatman, 2005). A research model developed as part of the 2003 California TOD travel characteristics study appears to confirm the importance of highway travel conditions. The model is presented and commented on, variable by variable, in Table 17-30. Note that many of the variables included in the model were either yes-no "dummy" variables or comparative ratios. An overall conclusion that may be derived from the model is that the probability of station-area residents choosing transit for commuting will be increased to the extent that localized congestion leads to slower highway trips while transit trip times manage to hold steady thanks to exclusive right-of-way or lanes (Lund, Cervero, and Willson, 2004a). 17-64
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Table 17-30 Binomial-Logit Model for Predicting Transit Mode Choice for Commute Trips by Station-Area Residents Variable Coef. T-stat Notes Travel Time and Patterns Comparative times: Ratio of travel 3.180 3.11 The more competitive transit time is time via highway network to travel relative to highway time, the higher time via transit network the probability of transit choice. Chained trip (1=yes; 0=no) -2.147 3.34 Chained trips are harder to serve with transit and, thus, residents that trip chain are less likely to choose transit. Regional Accessibility Job Accessibility via highways: -0.040 1.96 Better highway access to jobs leads to Number of jobs in 100,000s that can lesser probability of choosing transit. be reached via highway network within 60 minutes peak travel time Transportation Options at Workplace Flexible work schedules (1=yes; 4.194 7.39 Flexible work schedules can facilitate 0=no) synchronizing the workday with transit schedules. Free parking (1=yes; 0=no) -2.370 4.70 Having free parking provides an inducement to drive to work and thus makes transit choice less likely. Employer helps with vehicle -3.618 4.38 Similarly, having free tolls or fuel expenses (1=yes; 0=no) makes transit choice less likely. Neighborhood Design Connectivity levels at destination: 2.021 1.59 Pedestrian connectivity and proportion of intersections that are environment at the destination are 4-way or more important to transit choice because almost all transit trips ultimately become walking trips to reach the final destination. Demographics and Attitudes Auto ownership levels: Number of -2.976 5.21 Availability of a vehicle leads to a motorized vehicles per household greater likelihood of vehicle use and member 16 years or older lesser likelihood of transit choice. Transit lifestyle preference: Access 1.471 3.23 Residents that elect to live in a place to transit a top factor in choosing because it is close to transit are more residential location (1=yes; 0=no) likely to use transit. Constant -1.994 2.36 Number of Cases 726 Sources: Lund, Cervero, and Willson (2004a), "Notes" column by Handbook authors. 17-65