The US banned leaded gas for all cars on the road in 1996 but we phased it out much earlier. The Clean air act was passed in 1970 which created the EPA. The EPA set standards on how much carbon monoxide, nitrogen oxide, and hydrocarbons cars could have in their exhaust. To meet those standards automakers had to use catalytic converters and leaded gas destroys catalytic converters quickly. Because of this no new car introduced in the US since 1975 has used leaded gas. It was banned about 20 years later to let the old cars be operated through their normal lifecycle. We still use lead on n some aviation fuel.