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36 consecutive weekdays, one-day estimates were computed by where T = Vehicle Trip Ends and X = Number of Dwelling dividing the two 24-hour counts by two.) For all 17 TOD- Units. For the Wayside Commons projects, the corresponding housing projects combined, a weighted average trip generation ITE land-use category is Residential Condominium (ITE rate was estimated. (The ITE manual defines weighted aver- Code 230). The average trip rate for condominiums is 5.68 age as the sum of trip ends for all projects divided by the sum vehicle trips per dwelling unit on a weekday based on the of the independent variable, which in this case is number of experiences of 54 owner-occupied condominium and town- dwelling units.) The computed rates for TOD-housing proj- house projects across the United States (averaging 183 dwelling ects were compared to those found in the latest edition of the units in size). The best-fitting regression equation for condo- ITE manual for the equivalent land use (i.e., apartments and miniums is: condominiums) (ITE, 2003). Comparisons are drawn using the ITE manual's weighted averages as well as estimates de- Ln(T) = 0.85(X) + 2.55 (R2 = 0.83) rived from best-fitting regression equations. The degree to which there are systematic differences in estimated and actual where trip generation and parking generation rates of TODs are highlighted. The types of TOD projects for which there appear T = Vehicle Trip Ends, to be the largest discrepancies are identified. X = Number of Dwelling Units, and Additionally, results were cross-classified among sampled Ln = natural logarithm. projects in terms of distance to CBD, distance to the nearest station, parking provisions, and other factors including the Taking the (unweighted) average across the 17 case-study quality of walking environment (e.g., with or without adjoin- projects, TOD-housing projects generated around 47% less ing sidewalks). Multivariate regression equations that predict vehicle traffic than that predicted by the ITE manual (3.55 the trip generation rates of TOD housing as a function of trips per dwelling unit for TOD-housing versus 6.67 trips per these and other variables also are estimated. dwelling unit by ITE estimates). This held true using both the Lastly, the implications of research findings for various pub- weighted average ITE rate as well as the ITE rates predicted lic policies and practices are discussed. To the degree that TOD- using the best fitting regression equations. Results were quite housing projects exhibit below-normal trip generation rates, a similar in both cases. strong case can be made for using sliding-scale impact fees to The biggest trip reduction effects were found in the evaluate new TOD proposals. This might, for instance, result in Washington, D.C. metropolitan area. Among the five mid-to- lowering the estimated trip generation rates within a quarter high rise apartment projects near Metrorail stations outside mile of a station and with continuous sidewalk access and in a the District of Columbia, vehicle trip generation rates were mixed-use neighborhood by a fixed percent, such as 20%. more than 60% below that predicted by the ITE manual. There, 24-hour vehicle trip rates ranged from a high of 4.72 trip ends per dwelling unit at the more suburban Avalon project near Comparison of Vehicle Trip the Grosvenor Metrorail Station (and outside the beltway) to Generation Rates a low of around one vehicle weekday for every two dwelling TOD-housing clearly reduces auto trips in the four urban- units at the Meridian near Alexandria's Braddock Station. The ized areas that were studied. Below, results for both 24-hour comparatively low vehicle trip generation rates for TOD- periods as well as peak periods are summarized. housing near Washington Metrorail stations matches up with recent findings on high transit modal splits for a 2005 survey of 18 residential sites (WMATA, 2006). For projects within a Average Weekday Trip Comparisons quarter mile of a Metrorail station (which matched the Table 2.2 shows that in all cases, 24-hour weekday vehicle locations of all five TOD housing projects studied in the Wash- trip rates were considerably below the ITE weighted average ington metropolitan area), on average 49% of residents used rate for similar uses. [The comparable ITE land use category Metrorail for their commute or school trips. One of the proj- for 16 of the 17 projects is Apartments (ITE Code 220). The ects surveyed, the Avalon apartments at Grosvenor Station, average trip rate for apartments is 6.72 vehicle trips per also was surveyed in the 2005 study. The Avalon, which had the dwelling unit on a weekday based on the experiences of highest trip generation rate among the five projects surveyed in 86 apartment projects across the United States (averaging 212 the Washington area, had an impressively high work-and- dwelling units in size). The best-fitting regression equation school trip transit modal split in the 2005 survey: 54%. for apartments is: It is important to realize that high transit ridership levels and significant trip reduction in metropolitan Washington is T = 6.01(X) + 150.35 (R2 = 0.88) tied to the region's successful effort to create a network of
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37 Table 2.2. Comparison of TOD housing and ITE vehicle trip generation rates: 24 hour estimates. TOD Average ITE Rate (24 Hours) Regression ITE Rate (24 Hours) Veh. Trip Rate ITE Rate TOD rate as % of % point difference ITE Rate TOD rate as % of % point difference (24 hr.) (24 hr.) ITE Rate (24 hr.) from ITE Rate (24 hr.) ITE Rate (24 hr.) from ITE Rate Philadelphia/NE NJ Gaslight Commons 5.08 6.72 75.52% -24.48% 6.76 75.05% -24.95% Station Square 4.76 6.72 70.81% -29.19% 6.44 73.84% -26.16% Mean 4.92 -- 73.17% -26.83% 6.60 74.45% -25.55% Std. Dev. 0.22 -- 3.33% 3.33% 0.22 0.86% 0.86% Portland, Oregon Center Commons 4.79 6.72 71.30% -28.70% 6.53 73.36% -26.64% Collins Circle 0.88 6.72 13.08% -86.92% 7.22 12.17% -87.83% Gresham Central 5.91 6.72 87.95% -12.05% 7.68 76.95% -23.05% The Merrick Apts. 2.01 6.72 29.84% -70.16% 6.82 29.39% -70.61% Quatama Crossing 6.34 6.72 94.38% -5.62% 6.22 101.95% 1.95% Mean 3.99 -- 59.31% -40.69% 6.52 58.76% -41.24% Std. Dev. 2.42 -- 36.05% 36.05% 0.62 36.88% 36.88% San Francisco Bay Area Mission Wells 3.21 6.72 47.80% -52.20% 6.39 50.23% -49.77% Montelena Homes 2.46 6.72 36.57% -63.43% 6.81 36.09% -63.91% Park Regency 5.01 6.72 74.61% -25.39% 6.19 81.04% -18.96% Verandas 3.10 6.72 46.17% -53.83% 6.54 47.42% -52.58% Wayside Commons 3.26 5.86 55.68% -44.32% 6.00 54.34% -45.66% Mean 3.41 -- 52.17% -47.83% 6.39 53.83% -46.17% Std. Dev. 0.95 -- 14.27% 14.27% 0.31 16.66% 16.66% Washington, D.C. Area Avalon 4.72 6.72 70.21% -29.79% 6.31 74.75% -25.25% Gallery 3.04 6.72 45.25% -54.75% 6.66 45.66% -54.34% Lennox 2.38 6.72 35.41% -64.59% 6.38 37.29% -62.71% Meridian 0.55 6.72 8.24% -91.76% 6.34 8.73% -91.27% Quincey 1.91 6.72 28.49% -71.51% 6.31 30.34% -69.66% Mean 2.52 -- 37.52% -62.48% 6.40 39.35% -60.65% Std. Dev. 1.53 -- 22.76% 22.76% 0.15 24.06% 24.06% Unweighted Average 3.55 6.67 53.29% -46.71% 6.59 53.92% -46.08% Note: Fitted Curve Equation for Apartments: T = 6.01(X) + 150.35, where T = average vehicle trip ends and X = number of dwelling units. Fitted Curve Equation for Condominiums (Wayside Commons): Ln(T) = 0.85 Ln(X) + 2.55 TODs, as revealed by the Rosslyn-Ballston corridor (and slightly below the average rate from the ITE manual and a discussed in detail in TCRP Report 102: Transit Oriented bit above the regression-generated estimate from the ITE Development in the United States: Experiences, Challenges, and manual). Prospects). Synergies clearly derive from having transit- Also among the surveyed Portland-area apartments, notable oriented housing tied to transit-oriented employment and for its low trip generation rate, is The Merrick Apartments transit-oriented shopping. near the MAX light rail Convention Center station in the After the Washington, D.C. area, TOD-housing in the Lloyd District, across the river from downtown Portland: Portland area tended to have the lowest weekday trip gen- 2.01 weekday trips. Travel behavior of the residents of The eration rates, on average, around 40% below that predicted Merrick apartments also was studied in 2005 (Dill, 2005). by the ITE manual. The range of experiences, however, var- Based on a 43% response rate from 150 surveyed households ied a lot, from a low of 0.88 weekday vehicle trips per at The Merrick apartments, trip generation estimates can be dwelling unit for Collins Circle in downtown Portland to a imputed from that survey. The 2005 survey asked: "In the high of 6.34 for more suburban Quantama Crossing (only past week (Saturday January 29 through Friday February 4),
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38 how many times did you go to the following place from your per dwelling unit); Park Regency: 37% (5.01 daily trips per home in a vehicle, walking, bicycling, riding the bus, or riding dwelling unit); and Mission Wells: 13% (3.21 daily trips per MAX light rail? Each time you left your home during the week dwelling unit). is a trip." From household responses, an average of 1.42 daily Lastly, the two apartment projects near suburban com- vehicle trips per dwelling from The Merrick apartments was muter rail stations outside Philadelphia and the Newark met- made. Doubling this rate (assuming those who drove away ropolitan area of northeast New Jersey averaged weekday each day also returned) yields an estimated daily rate of 2.84 ve- vehicle trip generation rates roughly one-quarter less than hicle trips per dwelling unit. This is a bit higher than that found the number predicted by the ITE manual. This is an appre- in the tube count survey, but still substantially lower than the ciable difference given the relatively low-density settings of ITE rate. (Differences are likely due to several factors. These re- these projects and that commuter rail offers limited midday sults are based on objective physical counts whereas the 2005 and late-night services. survey results were based on a sample of self-reported re- sponses. Also, the 2005 study included weekend days whereas AM Peak Comparisons this study was based on middle-of-the-week experiences.) The 2005 survey also estimated that 18% of all trips made by resi- Table 2.3 compares recorded trip generation rates with dents of The Merrick apartments are by transit (both rail and those from the ITE manual for the AM Peak. In tabulating bus). For work and school trips, transit's estimated modal split the results, the one-hour period in the AM peak with the was 23%. A follow-up 2005 survey of The Merrick apartment highest tube count was treated as the AM peak. In most residents further indicated that transit is the primary commute instances, this fell between the 7 AM and 9 AM period. In mode for 27.9% of residents (Dill, 2006). general, patterns were quite similar to those found for the Another study further sheds light on the results for one of 24-hour period. As before, the greatest differential between Portland's surveyed apartments: Center Commons in east AM trip generation and ITE estimates were for TOD-housing Portland. This study's survey found a weekday rate of 4.79 closest to CBDs - notably, Collins Circle and The Merrick trips per dwelling unit for Center Commons, more than one- Apartments in the case of Portland, and the Meridian Apart- quarter below ITE's estimated rates for apartments. For a ments near the Braddock Metrorail station in Alexandria, thesis prepared for the Master of Urban and Regional Plan- Virgina. ning degree at Portland State University, a mailback survey of 246 residents of Center Commons was conducted in PM Peak Comparisons 2002, producing a response rate of 39%. That survey found that 45.8% of responding residents of Center Commons takes Table 2.4 shows the results for the PM peak. (The one-hour MAX light rail or bus to work. period in the PM peak with the highest tube count was treated As with metropolitan Washington D.C., Portland's success as the PM peak. This generally occurred in the 4 PM to 7 PM at reducing automobile trips around transit-oriented hous- period.) PM trip generation rates are generally higher than ing cannot be divorced from the regional context. High rid- the morning peak since commuter traffic often intermixes ership and reduced car travel at the surveyed housing projects with trips for shopping, socializing, recreation, and other stems from the successful integration of urban development activities. In general, PM trip generation rates for TOD- and rail investments along the Gresham-downtown-westside housing were closer to ITE predictions than the AM peak. axis. In Portland, as in Washington, TODs are not isolated Notable exceptions were the lowest trip generators. For islands but rather nodes along corridors of compact, mixed- example, the PM rates for Collins Circle and Meridian were use, walking friendly development. 84.3% and 91.7% below ITE predictions, respectively. For The San Francisco Bay Area also averaged vehicle trip the AM period, the differentials were 78.7% and 90.0%, re- generation rates substantially below estimates by the ITE spectively (from Table 2.3). manual. Among the East Bay TOD-housing projects studied, Montelena Homes (formerly Archstone Barrington Hills) Weighted Average Comparisons had the lowest weekday rate: 2.46 trip ends per dwelling unit, 63% below ITE's rate. A 2003 survey of residents of this proj- The summary results presented so far are based on un- ect found very high transit usage among Montelena Homes weighted averages, that is, each project is treated as a data point residents: 55% stated they commute by transit (both rail and in computing averages regardless of project size. The ITE bus) (Lund, et al, 2004). The 2003 survey found the following manual, however, presents weighted averages of trip genera- commute-trip transit modal splits (compared to this research's tion by summing all trip ends among cases and dividing by the recorded weekday trip rates): Wayside Commons: 56% (3.26 sum of dwelling units. Thus for apple to apple comparisons, daily trips per dwelling unit); Verandas: 54% (3.1 daily trips weighted average vehicle trip rates were computed for all
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39 Table 2.3. Comparison of TOD housing and ITE vehicle trip generation rates: AM peak estimates. Average Rate Regression Rate Veh. Trip Rate ITE Rate TOD rate as % of % Below ITE Rate TOD rate as % of (AM peak hr.) (AM peak hr.) ITE Rate (AM pk hr.) ITE Rate (AM peak hr.) ITE Rate (AM pk hr.) % Below ITE Rate Philadelphia/NE NJ Gaslight Commons 0.40 0.55 72.73% -27.27% 0.55 72.59% -27.41% Station Square 0.36 0.55 66.21% -33.79% 0.54 67.17% -32.83% Mean 0.38 -- 69.47% -30.53% -- 69.88% -30.12% Std. Dev. 0.03 -- 4.61% 4.61% -- 3.83% 3.83% Portland, Oregon Center Commons 0.25 0.55 45.45% -54.55% 0.54 45.90% -54.10% Collins Circle 0.12 0.55 21.26% -78.74% 0.56 20.74% -79.26% Gresham Central 0.59 0.55 107.07% 7.07% 0.58 102.10% 2.10% The Merrick Apts. 0.13 0.55 23.10% -76.90% 0.55 22.98% -77.02% Quatama Crossing 0.30 0.55 54.98% -45.02% 0.54 56.42% -43.58% Mean 0.28 -- 50.37% -49.63% -- 39.70% -60.30% Std. Dev. 0.19 -- 34.83% 34.83% -- 23.65% 23.65% San Francisco Bay Area Mission Wells 0.48 0.55 86.72% -13.28% 0.54 88.20% -11.80% Montelena Homes 0.17 0.55 31.43% -68.57% 0.55 31.30% -68.70% Park Regency 0.34 0.55 61.85% -38.15% 0.53 63.59% -36.41% Verandas 0.19 0.55 35.14% -64.86% 0.54 35.47% -64.53% Wayside Commons 0.21 0.44 47.35% -52.65% 0.62 33.50% -66.50% Mean 0.28 -- 52.50% -47.50% -- 50.41% -49.59% Std. Dev. 0.13 -- 22.53% 22.53% -- 24.88% 24.88% Washington Avalon 0.44 0.55 80.30% -19.70% 0.54 82.02% -17.98% Gallery 0.25 0.55 44.86% -55.14% 0.55 45.01% -54.99% Lennox 0.18 0.55 32.47% -67.53% 0.54 33.05% -66.95% Meridian 0.05 0.55 9.95% -90.05% 0.54 10.15% -89.85% Quincey 0.18 0.55 32.91% -67.09% 0.54 33.62% -66.38% Mean 0.22 -- 40.10% -59.90% -- 21.88% -78.12% Std. Dev. 0.14 -- 25.78% 25.78% -- 16.60% 16.60% Unweighted 0.28 0.54 51.30% -48.70% 0.55 50.64% -49.36% Average Note: Fitted Curve Equation for Apartments: T = 0.53(X) + 4.21 where T = average vehicle trip ends and X = number of dwelling units. Fitted Curve Equation for Condominium (Wayside Commons): Ln(T) = 0.82 Ln(X) + 0.17 17 projects combined for weekday, AM peak, and PM peak. rail-served metropolitan areas, such as Washington, D.C., (As done in the ITE manual, the weighted average was com- by as much as 50%. puted by summing all trip ends among the 17 projects and dividing by the sum of dwelling units.) Figure 2.6 summa- Scatterplots rizes the results. Over a typical weekday period, the 17 sur- veyed TOD-housing projects averaged 44% fewer vehicle The ITE Trip Generation manual reports summary findings trips than estimated by the ITE manual (3.754 versus 6.715). in a scatterplot form, with summary best-fitting regression The weighted average differentials were even larger during equations. Figures 2.7 through 2.9 show the best-fitting plots peak periods: 49% lower rates during the AM peak and 48% for the average weekday, AM peak, and PM peak periods, re- lower rates during the PM peak. To the degree that impact spectively. Linear plots fit the data points reasonably well, fees are based on peak travel conditions, one can infer that explaining over two-thirds of the variation in vehicle trip ends. traffic impacts studies might end up overstating the poten- The Merrick Apartments in Portland stands as an outlier, tial congestion-inducing effects of TOD-housing in large producing far fewer vehicle trip ends relative to its project size