Route 14 Extension and the Self-Selected Survey
A regional transit authority is deciding whether to extend Route 14 into the Northgate business park. Planners surveyed 1,200 current Route 14 riders during morning commutes; 68% said they would use the extended route at least twice a week. Only 264 surveys were returned. Route 14 currently carries 9,000 riders per weekday. The planning memo concludes that extending the route will raise total ridership by 15% within a year because 'most riders support the extension.' The extension would add 3.2 miles and require two additional buses, costing $1.1 million annually. Northgate employers report 4,000 daytime workers, but the authority has no data on how many drive alone or would switch. A board member notes that the survey was distributed only on buses, not at park-and-ride lots or online. The memo does not define 'support' or report a margin of error. The board must decide whether to fund the extension based on the memo.
1. Identify the central flaw in the planning memo's reasoning, and explain how it undermines the 15% ridership projection.
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The memo moves from '68% of returned surveys support the extension' to 'total ridership will rise 15%.' That inference has three gaps. First, the sample is not representative: it includes only current Route 14 riders, who already use transit, and only 264 of 1,200 surveys were returned (22%). Riders who are dissatisfied or who do not ride at all are excluded. Second, 'support' is not 'use'; the survey does not ask how often respondents would actually ride or what they would give up. Third, the arithmetic cannot support the projection: 68% of 264 is about 180 people. Even if each added two trips per week, that is roughly 360 extra rides per week-about 72 per weekday-while a 15% increase on 9,000 weekday riders requires 1,350 additional weekday riders. The memo has no data on Northgate workers' current modes. So the conclusion is not established.
2. What additional evidence would most strengthen the memo's conclusion, and why would it help?
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The most useful evidence would be a representative, revealed-preference study of Northgate workers and current non-riders-for example, fare-card counts from a temporary extension or a survey of all 4,000 workers with a high response rate. If it showed that at least 1,350 additional weekday riders would switch from driving alone, the projection would gain direct support. This helps because it addresses the two weakest links: whether support translates into use and whether the population that could generate new ridership is represented. It would still need to rule out induced demand from current riders and seasonal variation, but it is more probative than another survey of current riders.
3. Is the 68% support figure a necessary condition, a sufficient condition, or neither for the conclusion? Explain.
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Neither. The 68% figure is not necessary: ridership could rise by 15% even if fewer than 68% of current riders supported the extension, because new riders from Northgate could generate the increase. It is not sufficient: even unanimous support among a biased sample of current riders would not show that non-riders would switch. At most it is weakly relevant evidence about current riders' preferences. The board should treat it as a hypothesis to test, not as a conditional guarantee.