And they're more expensive than a slight curve.

Curves, too.

Speed bumps are disproportionally less effective on vehicles with larger wheels, which is the trend for SUVs and trucks these days. So while a compact or sedan may slow down for the bump, your average pavement princess will respect the bumps as much as they'll respect speed limits around schools. Which is to say, not at all.

Speed bumps slow me down far less than curves.

My car (small truck) is most comfortable going over bumps at 30mph... Not the 10 they expect you to slow to. At 10 you get bounced around, at 30 you get 2 separate bumps that are far less uncomfortable.

I call that really crappy design.