ATTENTION: Rules may vary in other countries, so we advice you to check the specific rules in your country, as we are only familiar with the rules in Denmark.
In Denmark, it is legal to use traffic alarms such as Saphe to warn about traffic controls etc. However, it is not legal to operate the mobile phone while driving unless you can do this without touching the phone, i.e. a Saphe traffic alarm actually gives you a legal opportunity to report and confirm alarms, as you are allowed to operate the cell phone via retrofit buttons on the dashboard - which the Saphe traffic alarm is.
You can read more about it here: The Council for Safe Traffic (content is in Danish).
There are also devices that can detect speed cameras and thus alert the driver, but these are not legal to use in Denmark, which is why Saphe uses reports from users to determine the positions of traffic alarms, which is legal.
There are countries where the rules are more complicated, e.g. in Germany the driver of the car is not allowed to use apps and the like to receive alarms for speed cameras, but passengers may.