What rights extend to personal vehicles on company property?

Yes, in this situation you would be able to require the employee to remove the objectionable material from their personal vehicle, but you may not always have that right. The federal National Labor Relations Act (NLRA) gives employees the right to engage in concerted activity for mutual aid and protection related to wages, hours, and working conditions (i.e. making a protest or complaining about low wages). Employees also have the right under both state and federal safety regulations to make a protest or complaint regarding reasonable safety concerns. Flying the Confederate flag in a personal vehicle on company property would not be protected activity under any of these laws, but if the content had a different message it might be protected. For example, if the employee sported a bumper sticker that said, “Proud to be a union thug,” an employer might find that objectionable but wouldn’t be allowed to require its removal because it would be considered concerted activity. Before requiring employees to remove material from personal vehicles on company property, it is best to review the issue and related consequences with an attorney.