E-Scooter Injuries and the Missing Denominator
After a six-month e-scooter pilot, the city of Riverton recorded 40 emergency-department visits for scooter-related injuries. During the same period, scooter trips rose from 10,000 to 80,000 per month. The city council president cites the 40 injuries and proposes an immediate ban, saying 'the pilot has made our streets more dangerous.' A transportation analyst notes that the injury count is not adjusted for the sevenfold increase in trips; the injury rate per 100,000 trips fell from 12.5 in the first month to 6.0 in the last month. The analyst also notes that 9 of the 40 injuries involved riders under 18, who were not permitted under the pilot rules, and that the pilot added 4 miles of protected lanes. The council must vote on the ban. The scooter company argues that the ban will push riders to cars, which have higher injury rates per trip. The council has no data on scooter-involved collisions with pedestrians.
1. Is the president's inference from '40 injuries' to 'more dangerous' justified? Explain the role of the denominator and base rate.
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Not as stated. The president compares 40 injuries with an implicit baseline of zero, but the relevant baseline is exposure. Scooter trips rose from 10,000 to 80,000 per month-an eightfold increase. The analyst's rate calculation shows risk per 100,000 trips fell from 12.5 to 6.0. Total injuries can rise while per-trip risk falls because far more people are riding. 'More dangerous' could mean more total harm or higher risk per trip; the president uses the former to imply the latter. The inference is also complicated by 9 under-18 riders violating pilot rules and by the addition of protected lanes. However, the rate data cover only two months, and pedestrian collisions are missing, so the president's conclusion is not established-but neither is the analyst's implicit 'it got safer' conclusion.
2. What further information would be most useful in deciding whether to ban scooters? Prioritize two pieces of evidence and explain why each matters.
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First, severity-adjusted injury rates per trip or per mile for scooters compared with bicycles, walking, and cars. A ban should be justified by comparative risk, not by scooter counts alone; if scooters are safer per trip than the cars riders would otherwise use, a ban could increase harm. Second, pedestrian collision and injury data, plus enforcement data on under-18 riding. Scooters impose externalities on non-riders, which the current 40-injury count omits. Hospitalization rates matter more than ED visits because a ban should target serious harm. These two pieces are more useful than another raw injury count because they answer 'compared with what?' and 'who bears the risk?'
3. The company's argument assumes something about rider behavior. What is the assumption, and is it sufficient to reject the ban?
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The company assumes that riders displaced by a ban will switch to cars, and that per-trip injury rates are the right comparison. The first is an empirical assumption: riders might walk, take transit, or stop making the trip. The second is questionable because car trips are often longer and more severe; a per-trip comparison can understate car risk per mile. The argument is not sufficient to reject the ban because it ignores pedestrian injuries, under-18 compliance, and the possibility that protected lanes and enforcement could reduce harm without a ban. It is relevant, but it needs substitution data and severity-adjusted comparisons to become decisive.