The Mille Miglia endurance race from Brescia to Rome and back took place 24 times between 1927 and 1957 (13 before World War II and 11 after), pitting Gran Turismo sports cars made by Ferrari, Maserati, Porsche and Alfa Romeo against each other on the original figure-eight shaped course. The race was banned in 1957 following the crash of a Ferrari that took the life of its driver, and did not resume until 1977, albeit more as a way to showcase pre-1957 cars, rather than really race them.
Not that the entrants don't take today's Mille Miglia seriously. Even though they are at the wheel of cars worth millions of dollars, they will accelerate to take the lead–all in good fun, of course. Noteworthy is that if your wealthy clients are engaged in any of these events, or frankly, any collector car event involving a stopwatch, there are exclusions in their insurance policies that apply.
It is critical that you, their trusted advisor, make certain that the insurance broker for the collection is aware of the event and makes the necessary adjustments to the policy.