We review milestones in the history of increases in speed limits and travel speeds ("speed creep") and risks for road deaths and injury. Reduced speed limits, speed-camera networks, and speed calming substantially reduce these tolls in absolute numbers-a trend that is apparent in the United Kingdom, Australia, France, and other countries, but not in the United States, which has raised speed limits and does not have speed-camera networks. Newtonian relationships between the fourth power of small increases or reductions in speed and large increases or reductions in deaths state the case for speed control. Speed adaptation and the interaction between speed and other determinants of injury risks, including congestion and countermeasures, enter into these relationships. Speed-camera networks and speed calming lead to large, sustainable, and highly cost-effective drops in road deaths and injuries and should target entire populations, not merely high-risk subgroups or situations. Yet, there are major barriers to preventive strategies based on the discovery that speed kills. Modal shifts from speed on roads to speed on rail, lower maximum vehicle speeds, and speed-camera networks are required for progress toward Vision Zero-the goal of no road deaths-through Killing Speed. The human cost of the delay in killing speed in the United States may be as high as 20,000 lives lost per year.