Drivers who don't cut their engines when waiting aren't just wasting fuel: they're breaking the law and risk being fined.
So there I was, parked up with my engine off, waiting for my daughter to come out of school, while the driver next to me kept his engine running.
Maybe he'd spotted his own child on the way out and was keen to make a quick getaway.
But no, 15 minutes later, I was the first to drive away leaving him still sitting there with his engine growling away, making a racket, wasting fuel and polluting the environment.
Why? What's the point? Surely he wasn't so lazy he couldn't just turn a key in the ignition to cut the engine?
Leaving an engine idling is an offence
"Stationary idling is an offence under section 42 of the Road Traffic Act 1988," says Jeanette Miller, a managing director of Geoffrey Miller Solicitors.
The Act enforces rule 123 of the Highway Code which states: "You must not leave a vehicle engine running unnecessarily while that vehicle is stationary on a public road."
And doing this can incur a £20 fixed-penalty fine under the Road Traffic (Vehicle Emissions) Regulations 2002. This goes up to £40 if unpaid within a given timeframe.
Of course, it doesn't mean you've got to cut your engine at every red light: you are allowed to leave your engine running if you're stationary in traffic or diagnosing faults.
Some taxi drivers, however, seem to think they've got their own set of rules.
Confused.com's former content editor, Naphtalia Loderick, faced problems with noisy taxis when she needed early morning lifts while working 6am shifts in her previous job.
"My taxi would turn up early and sit outside my house for 10 to 15 minutes with its lights on and engine running," she says.
"I had to get up even earlier simply to go out and ask them to cut the engine. They seemed oblivious to the noise, let alone the environmental pollution they were causing."
Private land exemption
The rules over stationary idling apply to public roads so if idling in the school car park would be okay, but it could prove costly if you're caught idling on the street.
Penalty Charge Notices (PCNs) are issued by councils rather than the police.
Islington Council in London mounted what's thought to be the first crackdown of its kind on vehicles churning out "unnecessary pollution".
Enforcement officers are out on the roads in north London tackling hotspots to educate drivers about the advantages of turning off their engine.
"An idling engine can release as much pollution into the air as a moving one," says Islington councillor Claudia Webbe.
"Cutting the engine while stationary reduces both the harmful pollutants being released and saves on fuel."
Turn off engine for waits more than one minute
"Engines should be turned off for waits of more than one minute," says Professor Stephen Glaister, director of the RAC Foundation.
You can then turn it back on without using the accelerator, which uses almost no fuel in the process, he adds.
"Between 5% and 8% of fuel use occurs while idling," says Professor Glaister.
Based on the average annual fuel bill of £1,600, this means wasting more than £100 a year as well as affecting air quality.
Some manufacturers like Volkswagen now offer "stop-start" technology as an option on new vehicles.
This means the engine automatically cuts as you come to a standstill and restarts when you apply the clutch.