Older carbureted engines would use a lot of fuel to start up, so there was a “5 minute rule.” If you were stopping for less than 5 minutes, it would use more gas than letting the car idle. I have no idea if this is true or not scientifically, but that was what people said. These days though, that’s not the case at all. They build cars now that stop the engine at a stoplight and start it back up again 10 seconds later because it’s more efficient with modern engines.