

They don’t have to lose full control. They could be following too closely and swerve onto the sidewalk to avoid a collision with a car and end up striking a pedestrian.
Speeders will stick out even more making them easier to spot in traffic. The reduced speed limit should also be accompanied by lane narrowing, speed humps, and other traffic calming techniques.
The reduced speed limit should come with traffic calming relative to the speed limit. If a through road has a 20 MPH zone that area should have lane narrowing and at least raised crossings.