Driving a bus is no game of “Mario Kart,” Metrobus operators will say. In addition to making sure riders pay the fare, bus drivers grapple with avoiding cars, buses, bicyclists and pedestrians. Now a video of one unnamed Metrobus driver on the 52 Line headed from Takoma to L’Enfant rail stations highlights the challenge of what happens when a 4-year-old child runs into traffic. The 12-second video, provided by the transit agency, was shot just after 4 p.m. on April 20 by one of the new cameras installed in all Metrobuses to help train drivers.