The money to build one speed bump will pay for two bicycle lockers. A speed bump only slows down vehicles over a small section of a single road. The expense would be enormous. Even people who aren't poor, but who choose not to own an automobile, are forced to pay for the repair of streets that others damage. Then, some of the proceeds could be used to provide more traffic police to enforce the speed limit, without inconveniencing nondrivers.
Speed bumps solve the wrong problem.
And the quicker, the better.