"And it affects us all with insurance premiums and, obviously, the most serious thing is that they can kill themselves or somebody else."
Punishment or training
He helps run a local project called Kickstart, designed to get teenage riders caught breaking the law to swap punishment for road safety training.
Out on patrol he pulled over 16-year-old Sam Lewis from Yatton, just outside Bristol.
30 miles per hour doesn't sound very quick - depends how you can handle your bike
Moped rider Sam
It was nothing as serious as being unlicensed or under the influence - PC Greenhalgh just wanted a word with him about safety after Sam was driving a bit too close to the car in front of him and seemed a bit too keen to overtake.
Because it was a minor offence Sam was offered the chance to go on the course, which he agreed to.
He told us he sees all sorts of dangerous, illegal riders out there.
"We see them going down tracks," he said, "down lanes where they shouldn't be riding.
"People just going round with their feet on their seats trying to pull wheelies and stuff like that, just stupid really.
"Then they come up beside us, beep their horn and try and race and stuff and we just tell them where to go."
The engines on mopeds are only 50cc and they're limited to around 30 mph.
Even at that speed Sam says if people aren't in control of their machine it can still be dangerous.
"30 miles per hour doesn't sound very quick - depends how you can handle your bike," he explained.
"But if you can't handle your bike very good then you're going to be knackered."
Nearly 500 teenagers were killed or seriously injured on mopeds in 2007.
There's no way of knowing just how many teenagers are currently riding illegally - that is, unlicensed and untrained - but the Institute of Advanced Motorists has told Newsbeat the numbers are clearly in the thousands, and that the problem's getting worse.
They want a major police clampdown, but that's causing a row because senior officers claim the problem of illegal moped riders isn't widespread.