Scaling a Pedestrian Pilot in Marlow
The town of Marlow is considering making a 12-block downtown corridor car-free. Last year, a six-month pilot closed 3 blocks to cars. During the pilot, sales at businesses in the closed blocks rose 8 percent, while sales citywide rose 2 percent. Pedestrian counts on the closed blocks rose 30 percent. Two businesses in the pilot area closed, and delivery drivers reported 15-minute detours. The council's proposal cites the pilot as proof that a permanent, corridor-wide closure will 'revitalize downtown retail.' The council has not commissioned a study of surrounding streets, and the pilot ran from May to October, including a popular street festival. A residents' group argues that the pilot harmed elderly and disabled residents who rely on cars.
1. Identify the unstated assumption in the council's inference and explain why it matters.
Read the worked answer
Unstated assumption: that the pilot's 8% sales gain was caused by the closure and will scale from 3 blocks and 6 months to 12 blocks permanently. It matters because the evidence is an association, not a controlled comparison. The pilot ran May–October and overlapped a street festival, so at least part of the gain may be seasonal or event-driven. Citywide growth of 2% is a crude counterfactual: the pilot blocks may differ from the city in foot traffic, rents, or store mix. The two closures and 15-minute delivery detours are costs the council's claim ignores. Scaling from 3 to 12 blocks could also displace sales from surrounding streets rather than create them. Without addressing these, 'proof' is too strong a word.
2. Write a short argument evaluation that assesses the evidence without taking a side on whether the corridor should be closed.
Read the worked answer
The council's evidence is suggestive but not sufficient. The 8% versus 2% comparison is better than no comparison, yet it lacks a matched control: nearby unclosed blocks would show whether the gain was diverted trade. The 30% pedestrian increase measures activity, not spending, and foot traffic can rise while sales per visitor fall. Two closures in six months is a warning about fixed costs, not a decisive refutation. The festival and favorable months are plausible alternative explanations. A fair evaluation should ask for block-level sales before, during, and after the pilot; data from surrounding streets; and delivery times for disabled residents. The residents' equity concern is relevant but separate from the causal claim. The proposal should be tested with a staged expansion, not treated as proven.
3. Which additional piece of evidence would most weaken the council's claim, and why?
Read the worked answer
Most weakening evidence: sales at comparable nearby blocks outside the pilot rose by about 8% over the same months. That would undercut the claim that the closure caused the gain, because the pilot area would not outperform its neighbors. It would suggest either a citywide trend or displacement. A second strong candidate is week-by-week data showing the 8% was concentrated in festival week. Between these, the nearby-block comparison is stronger because it tests the counterfactual directly and covers the whole pilot period. It still would not prove the closure was harmful; it would only remove the main support for 'revitalize.' The council would then need a different design, such as randomizing which blocks are closed.