In the UK during the early era of cars, the Automobile Association used to send boys out with bicycles to warn drivers about speed traps. This was challenged in court, obstruction of justice or some such, so the AA simply inverted the scheme. The boys were now told to always salute cars to signal that everything was okay, but wouldn't salute if there was a speed trap ahead. It was reasoned that the law couldn't compel the boys to salute. Apparently they kept this up for a few decades, before eventually deciding that speed limits were generally reasonable.
If you drive around in EU then be aware that the law is different depending on the country. Schweiz for example do not allow to use or sell databases that has the location of speed cameras. In Germany you are not allowed to use apps that warn you of it. You are also not allowed to use your car lights to warn other drivers, but you can use hand signals. They are however allowed in other places like Belgium, Neitherlands and Spain.
As someone that lives and works on the DACH region, you just have to listen to German and Swiss radios, they do tend to point out radar spots, regardless of those laws, so dunno how they get their permissions to do so.
I use OsmAnd and it alerts me to pedestrian crosswalks. My wife uses google and it alerts her to speed enforcement. Interesting difference in priorities.
What are the benefits of crosswalks alerts? Usually, there are lots of signs leading to, and at the crosswalks. This is the opposite for fixed and mobile speed traps, they'd rather it be a surprise to drivers.
The benifit is adding another layer of safety to reduce risk. In the swiss cheese model of accidents, each safety measure is a layer of swiss cheese, which has holes through which accidents may go through. But if you stack up a lot of cheese, the accident has to thread a path through several different holes and that is less likely to happen.
