The road I live on is a 30mph residential road but it is also an A-road with signalled junctions.
Unfortunately frequently I will hear cars and motorcycles accelerating away from the lights at full bore, 1st-redline, 2nd-redline, 3rd-redline. Both cars and motorcycles hit 100mph along this 30mph road, I hear/see it several times per week.
This isn’t my road but physically it is similar enough for you to get an impression of it.
The 100mph maniacs happen a few times per week, 60mph a few times a day, 50mph a few times per hour, probably half the cars are doing 40mph, 30mph is the minimum.
Due to this the police park their mobile speed camera van outside my house a few times per month. The put it on a nice straight part of the road looking in the direction of a bend about 500m away. Typically what happens is cars going past the van in the direction of the bend will flash their lights at cars coming around the bend towards the van; getting flashed at by a stranger for no apparent reason is the universal warning for mobile speed camera.
I find that as a vulnerable road user whether on foot or cycle that any efforts I make to encourage drivers to slow down and met with ignorance or derision, but drivers seem to trust this universal indicator enough that if a fellow motorist flashes their lights they will dutifully slow down to the speed limit.
A couple of years ago Lancashire Police had a bit of a go at drivers who did this, handing out £30 for misuse of headlights, Daily Mail readers did not like it, the best rated post on the article with almost 2,000 up votes was
Surely the flashing of lights causes the oncoming vehicles to slow down, and isn’t that the point. So, far from committing an offence, the drivers are doing a public service, but without the revenue raising.
The worst rated post on the article with over 800 down votes was
Speeding lunatics should be caught and punished. As should those using their headlights contrary to the provisions of the road traffic act. The end.
The link is for evidenciary purposes only, there’s no need to click it, you won’t learn anything http://www.dailymail.co.uk/news/article-2157763/Drivers-fined-flashing-headlights-warn-oncoming-cars-police-speed-trap.html
What use can we draw from these points?
Well, firstly that if you flash your headlights to warn other road users of a speed trap you may be committing an offence and I would not condone anyone committing a road traffic offence.
Secondly, that drivers who flash their lights to warn others of danger spots are considered by Daily Mail readers to be “doing a public service” with a ratio of 1966:63 in support of flashing.
Now bearing in mind that you’re not allowed to flash your lights to warn about a speed trap because you could get fined, we might come to the conclusion that you can only flash your lights when there are no speed traps, but by causing the other driver to slow down in a potential danger zone you have “done a public service” and can go about the rest of your day feeling very happy with yourself.
Bearing all this in mind, would it be wrong of safety campaigners to take advantage of the position of trust embodied in being a fellow motorist to encourage other drivers to slow down for danger zones, but remember NOT to flash if there really is a speed trap, because that would be naughty?