Participants must purchase a travel medical + evacuation insurance plan from World Nomads (or equivalent company).
With World Nomads, choose the Explorer Plan. This covers motorcycle riding in Europe. (Cost varies by your age and state of residence. For me, a 46-year-old from Michigan, it would be $167.)
If choosing another company, ensure it covers an equivalent medical and evacuation of the Explorer Plan benefits, including on-road motorcycle riding in Europe.
It is recommended to purchase the insurance soon after booking the tour and flights.