A summary of your state EPAMD laws is a good idea if you are not disabled (and even if you are.) I also carry a summary of relevant parts of the ADA.
http://forums.segwaychat.org/showthread.php?t=25902 You are welcome to copy this.
If you are nervous about the whole thing, why not program the DOJ Help line number into your cell phone. Just reaching for it may resolve the problem. <G> You can always ask the DOJ about the issue and then ask them to explain it to the person who is hassling you...
ADA 2010 makes it quite clear that they cannot ask anything more about your medical conditions than "Are you disabled?" or ""Do you have a medical reason for using this?"
They are also required to accept your state issued disability card as sufficient proof--and failing that, they must accept your verbal assurance if there is no visual evidence to the contrary. (You are using the Segway, for Pete's sake! Now if you were turning cartwheels...)
There should be no need whatever to carry that prescription. It is none of their business...
Firm courtesy is your friend when issues arise. You do know what the law says and if they know that you can--and will-- file a formal complaint, it may help the situation. Then do follow up if necessary. It doesn't hurt to contact supervisors/ city/ state/etc officials, also newspaper and TV etc to resolve the issues if you can. Then if not, it is your responsibility to use the DOJ resources for the benefit of the rest of the disability community. If we don't, who will??
These issues can be irritating or much more than that, but they are also an opportunity for educating those who have authority to enforce laws. It pays to remember this always. If you can, it wouldn't hurt to have a friendly conversation with small town police. Give them a copy of your handout, offer to give them a ride or at the very least demonstrate how well the Segway controls. Prejudice is being down on something you are not up on!