Any given person might only experience a single flooded roadway or two in their lifetime. But that doesn't mean that there aren't tens of thousands of people exposed to flooded roadways every year. Something can be individually uncommon and yet frequent in absolute terms.
If you live in a city like Atlanta that gets significantly more rainfall that Seattle but concentrated into fewer rainy days, you’ll see flooded roadways multiple times per year.
It’s funny as someone from the North East who’s always lived around rivers I assumed uncommon means once or twice a year. Not disagreeing they are uncommon, nor suggesting driverless cars are never going to happen. Just marveling at how different two individuals understanding can be of the same word.