Racism and hate. I'd love to address drivers tailgating (#1 cause of accidents, so easily preventable), or hating cyclists (every person who uses a bike instead of a car means less road congestion, less waiting, not more).

But I think addressing racism would have a bigger impact right now - we're genetically the same, cultural differences are compatible, bad journalism and social media prods us to get angry, treating others like shit only makes others do the same, etc.