Disclaimer: I've never hunted SD in my 50+ years of hunting and have no pending plans to do so. Not that I have any problem with SD, it's that I have more than enough hunting opportunities here in ND so I see no reason to lay out the bucks to hunt an hour or so south of my place.
That being said:
Roadside counts are like any other survey or research project, accuracy is totally dependent on the people gathering the facts and how those facts are utilized/interpreted. I've seen far too many studies in my time where the researcher or entity has a hypothesis, then looks for facts/trends to support their theory in the data analysis. Road side counts are no different. interesting off-season grist for discussion, but in the end worth about as much as the paper they are written on. Probably more worthwhile to look at anecdotal evidence from locals on forums like this.
Anyone who hunts out of state (over the years I've done my share of big game hunting in other states & Canada), know that the state or province fleeces the non-resident out of as many dollars as they possibly can through application fees, mandatory stamps, etc. Some states, notably those in the west, have NR regulations so complex & confusing and it's so expensive to apply & actually draw that companies have sprung up that will do it for you, for a fee. I don't blame the states really, they have a commodity with value so want as much revenue generated by it as possible. The fact that this often prices the average hunter out of contention is just the way it is. SD is no different.
The bottom line is that if you want to hunt what a particular state has to offer, save up or get out the checkbook (yeah, I know, what's a checkbook?) or stay home. Doesn't do any good to bellyache about it.