You mention in your reply that the officer cannot or would not bring charges against the offender due to the fact that they didn't "see" the event. Well... They could issue a citation and make you a witness if your willing to go to court and testify. Many drivers in accidents are charged with traffic violations even though the officer "didn't see" the event. Perhaps your willing to go to court and testify to your observations, that will help convict this bad driver of some violations. With enough points and violations, theres no doubt that their CDL privledges will be suspended. You should speak with the deputies and troopers that patrol that area and inform them of the violations. Most of them have ticket machines and enjoy writing tickets. Troopers, will write tickets to anyone. Deputies have a history of writing citations to whoever the good sheriff allows to my understanding.
You made these calls to various agencies. Why not film the violation if its reoccurring and send a copy of the video to them. There is no shortage of videos on youtube of "bad driving." Show a pattern of abuse, and explain what the problem is. Perhaps your idea's and investigation will be enough to get the roadway/issue changed. It could be a design flaw or simply needing to add a traffic control device such as a stop light or traffic flow sign. You mentioned its an unsafe route. In that case, perhaps banning trucks and buses might be an option. You have to explain why its unsafe and make your case using photo/video/personal testimony and evidence.
Finally, follow up with these people. Your issue might take a back seat to all other issues, and your simply another "irrate citizen" just calling in and "venting." With that being said, letters to the editor on you taking notice to the problem, you notifying them and their follow up can sometimes be enough to cause for change. They hate public embarassment. Let them know you demand change, and stay on them. If they fix the issue, feel free to write a letter to the editor in praise of them. If they fail to perform or ignore/pass the buck to someone else, slam them of course in your letter to the editor/followup on here.