The head of the National Transportation Safety Board expressed concern Wednesday about the safety risks that heavy electric vehicles pose if they collide with lighter vehicles. The official, Jennifer Homendy, raised the issue in a speech in Washington to the Transportation Research Board. She noted, by way of example, that an electric GMC Hummer weighs about 9,000 pounds (4,000 kilograms), with a battery pack that alone is 2,900 pounds (1,300 kilograms) — roughly the entire weight of a typical Honda Civic. “I’m concerned about the increased risk of severe injury and death for all road users from heavier curb weights...