On the last Friday of every month, hundreds of bike riding enthusiasts take to the streets for Critical Mass in Porto Alegre, Brazil to raise awareness of cycling in a city dominated by motor vehicles -- but at their most recent event, the unthinkable happened. As the lively group of cyclists pedaled together down the street, one disgruntled motorist decided to accelerate through the crowd, running down dozens of riders in a disturbing hit-and-run.