I don't think radar guns are any kind of answer for this. I don't know that the telemetry that is displayed from these cars is entirely accurate either. What is completely accurate is the equation of speed = distance / time. Sure, they can go over the pit road speed limit for a brief time after passing the last timing line before their pit box. That's because they then park in their pit box. If they want to keep these guys/gal from speeding on pit road, put a timing line in at the beginning of each pit box and the pit lane exit. Problem solved. They wouldn't be able to speed in to or out of their pit box.
The other option is to just leave things as they are so we all have something to talk about.
Personally, I like that they are able to do what they do now. Picking the pit position is part of racing. Picking a pit just before a timing line is an advantage earned by their qualifying position. I like that.