I was just doing my duty of using arm signals to slow a few drivers down, who I thought were going too fast and it was coincidentally, just around the corner from a speed camera van.

Next thing I know a group of three of our finest pull up in an unmarked car and tell me that I am facing a possible arrest. Ten minutes later and a few questions and they let me off with a warning.

Yeah, I know 'it was a cool story bro' but I thought I best warn others, they take great offence to having their revenues reduced, even for just a few minutes.

Crime prevention = Bad
Punishing people after they have committed the crime = Good

I've always been bemused by that. It's a camera there for safety, so the road must have a dangerous point. You are merely recognising the same danger and alerting users that the road is dangerous at that spot and hadn't even seen a camera.

It is ridiculous, and morally wrong that the police do this.

This is nothing new, I remember in the late 60s or early 70s the police would put a radar trap up near me on what was called The Mad Mile on the A20, one day a man was sitting in the layby prior to the speed trap holding a big warning sign, he was soon pounced upon by a squad car full of police.

What evidence (of your supposed crime) did they present you with?

How could he tell they really were speeding? They may not have been, in which case what obstruction would he be causing? I would have thought his intent is immaterial in that instance.

Playing devil's advocate, there are three things you could potentially cause you to want to warn other motorists.

1) To stop them being caught (illegal)

2) To warn them that they are entering a dangerous bit of road (bit tenuous)

3) To warn them of the hazard caused by the speed van's presence...

Ok, I'm clutching at straws here but hear me out..
2009 ish ... somebody I know was driving a up the a9, doing 60mph - speed limit for the van on that road. A Skoda overtakes him, and suddenly veers of the road into a light pole, dies while the van driver is holding his head straight. Only explanation I can see for why it happened is the fixed speed camera right at the crash spot, the the driver saw it, panicked, locked up and ended up the road furniture.

2012, a82 loch lochy side. I was in a car with somebody who was overtaking a long line of cars. He sees a speed van in the distance and panics, starts braking without thinking about how he is going to complete the overtake. I say something along the lines of "you're caught already finish the fking overtake for fks sake!!!!!", and we go about our day. In the end he never got done for it.

While none of these incidents should have happened if people had their heads on straight, to me they demonstrate that a speed camera in itself causes a hazard. I'm not suggesting that they speed cameras are dangerous killing machines, but they are a potential hazard.

...I'm not sure that argument is going to work on a traffic cop somehow though!

Silly me, I thought the purpose of the police was the prevention as well as detection of offences/crime. Since the mantra is safety (the S in SCP) then one might be forgiven for thinking that it would be public spirited of the OP to get people to slow down. Getting all heavy about it suggests a certain degree of hypocrisy. Especially given the active encouragement given to MoPs to engage in local Speedwatch campaigns/activities. To me that is simply double standards.

Effectively what they are saying is "What you're doing hasn't received our 'kitemark'. Ipso facto your end product must be dodgy/unsafe."

Exceeding the posted speed limit is not a criminal offence.