Is it better to idle or turn the engine off when waiting?

Idling burns roughly a quarter to a half gallon of fuel per hour depending on engine size, and it delivers zero miles. The persistent counterargument, that starting an engine consumes more fuel than idling and wears the starter, comes from the carbureted era when cold starts required a rich fuel mixture and starters were less durable. Modern fuel-injected engines use a negligible amount of fuel to restart, and the Department of Energy's guidance is that turning the engine off beats idling beyond roughly ten seconds. The clearest evidence that the industry agrees is that automakers now build automatic stop-start into ordinary vehicles specifically to shut the engine at every red light.

The exceptions are worth stating so the rule holds up. Do not shut off in traffic where you need to move immediately, and do not do it repeatedly in stop-and-go where you would restart every few seconds. Diesel engines and turbocharged engines under heavy load benefit from a short cooldown before shutdown. Extreme cold changes the calculus for comfort and for defrosting, which is a safety consideration rather than an engine one. Beyond fuel, idling produces emissions at ground level where people are standing, which is the reason many jurisdictions have anti-idling ordinances near schools and hospitals, and why school bus idling in particular has been a target of public health regulation.