First of all, the existing 59 is not an unsafe road in and of itself. Most of the accidents that occur on this road are due to driver error. This indicates that the highway was not poorly designed. It just means that the design is inadequate for the job it is currently performing. The question that now stands is: if a new freeway is built away from the existing road, would the existing road be adequate for the remaining traffic? The answer is, in my mind at least, yes. K-10 was replaced by a freeway a long time ago, and the road the remains certainly handles it's job, as there haven't been any complaints about old K-10 recently. (except for the fact a few bridges have had to be replaced by the counties). Also note Douglas County Road 1055 (which runs between Lawrence and Baldwin) — it is not up to the same design as current 59 or old K-10, but also handles its job well without incident.
To make the road a little safer, reasonable traffic calming measures could be implemented once the road is “demoted.” For instance, stop signs could be added at intersections with the major numbered county roads, as well as US 56. Even roundabouts would not be out of the question — in fact, KDOT may help out so that they can determine if a rural roundabout can work.
That argument could be used by anyone that gets “Frontage roaded.” The short answer is that emergency crews are trained to know how to get where they need to go as fast as they can. The time lost due to increased miles traveled can be made up by the ability to travel faster down the freeway.
Unfortunately, widening the shoulders takes more effort than most people probably realize.
Between Pleasant Grove and the Franklin county line, the shoulders are practically non-existent. What you do have is a drop-off to the drainage ditches. The roadbed would have to be reprofiled completely in order to do what you suggest — that means tearing up the roadbed and giving commuters nightmares. Since KDOT feels that a freeway is going to be needed anyway, it is a waste of resources. The existing profile is quite suitable for a county road (see above)
Even with an unlimited budget, police officers cannot be everywhere at every time. Although the state police makes every reasonable attempt to crack down on speeders, there is no guarantee that speeders will drop out of sight. This is why traffic engineering has a basic principle of self-enforcement — that is, the roads are designed so that ~85% of the people are comfortable driving at or near the posted limit.