The extra risk is slowing or stopping when people don't expect it. Unexpected things cause collisions. There's also extra risk in creating an obstruction, which

The extra risk is slowing or stopping when people don't expect it. Unexpected things cause collisions. There's also extra risk in creating an obstruction, which creates risk for other drivers. There's no doubt that driving is inherently a terribly risky activity. But I also don't think that pretending that taking your foot off the gas on a highway is a risk-free activity is particularly helpful.