Sheila never says that the problem Robert mentions is less serious than she thinks. She just mentions another problem that could arise from Robert's proposed solution.

This is correct. Sheila points out that, if the city implements Robert's proposed solution of installing speed bumps, this could lead to drivers losing control of their vehicles.

Sheila does not propose an alternative solution to the problem Robert identifies. She just rejects Robert’s proposed solution based on its potential consequences.

Sheila never concedes that Robert's solution would work, whether for the reasons he proposes or for other reasons.

Sheila does not claim that Robert’s proposal would have no net effect on the problem. Her argument suggests that it would have a net effect that is different from what Robert expects: potentially making the situation more dangerous, not less.