Wooden covered bridges often had signs that said "Horses and cattle are not allowed to proceed at a gait faster than a walk." and then listed a stiff fine of ten dollars for anybody who they caught speeding across their bridge at a faster pace. Here is an example from the sixteenth street bridge in Pittsburgh PA, a wooden covered bridge, that burned down in 1919.
http://www.clpgh.org/exhibit/images/gif/16thf.gif Unfortunately, that is the biggest photo of that sign I can find. The print is big enough, you can use your browser to enlarge it by 200 % or 400 % and still be able to read an early example of a speed limit sign.