If anything, Gumball is a proof that it can be done safely. There are hundreds of contestants every single year and statistically it's one of the safest motorsport-events in the world.
Actually I think the bar for safe motorsports events is far higher than public roads. Nürburgring is considered one of the most dangerous and deadly courses to race on and it has had 68 fatalities since 1928. Per mile driven I believe that is probably an order or two of magnitude below the fatality rate of most highways, let alone ones that have been around since the 30s.
I don't particularly care to crunch the numbers or look up the figures, but I am going to go ahead and guess that one deadly incident for hundreds of drivers driving several thousand miles, for several years, is in the general ballpark of the standard fatalities per mile in America. Doing some quick "fermi-math" here, I'm guessing it is about the same.
Again, that is irrelevant for this case, where the guys racing are out on the street with its curbs, lamp posts, uneven paving, manholes, etc, and where they choose to be on the same 'track' as people who aren't volunteering to be in such a racing condition, and who aren't all, at the moment, physically or mentally prepared for it (in many cases, they aren't even capable of being physically and mentally prepared for it. Not everyone is born a racing driver, and those that are, grow old, too)
Formula one is relatively safe nowadays because all cars are safe, the drivers have superhuman driving ability, are physically prepared, and wear safety gear, because the circuits are smooth and without things you can hit that will kill you, and because medical assistance is close by when needed.
