Not necessary, because Sheila’s argument doesn’t rely on any comparison between Crownsbury and other cities.

Not necessary, because Sheila’s argument is concerned with whether Robert’s proposal is a good idea. Whether the proposal was intended to address the problem Robert had in mind doesn’t matter, because Sheila can still argue that the proposal is a bad idea because of certain negative effects she believes will occur as a result of it.

Necessary, because if it were not true — if there would NOT be any drivers who would not slow down to a safe speed (in other words, if every driver would slow down to a safe speed) — then the fact people who are driving too fast can lose control when they hit a speed bump would be irrelevant. Those drivers wouldn’t exist in Crownsbury, so the problem Sheila fears would not actually occur.

Not necessary, because Sheila’s argument isn’t based on the risk to pedestrians crossing the street (which are the people Robert was concerned about). Her argument is based on the risk to speeding drivers.

Not necessary, because Sheila’s argument concerns Robert’s proposal, which is about residential streets. So Sheila doesn’t have to have any opinion about whether the problems Robert describes occur on nonresidential streets.