The best Apps are always visible at any roads.
A round metal plate on a pole white background red surrounding an two digits often variing by country and road conditions between 20 and 130 in ten km steps.
The next one is the angled metal plate telling you a village or city name you now enter. Here you need to inform yourself what ist the legal standard limit beside any other signposting, in most cases in Europe 50 km/h. The end of such zones are sigen with the same metal plate and name but crossed with a red bar.
Then you need to inform yourself regard standard limits on country roads and highways of the country you enter. These limits vary too.
And finally the last one is discipline.
Sorry to write this.
But I drove now more than 50 years cars, short time a truck and also for few years a motorbike. 12 years I was on the road in several countries in Europe by my job for about 40-50.000 kilometers per year and more.
I was catched by cameras/radar 2 times 1980 when I was in a speach with my passenger not recognizing the limit, 100 DM fine.
and in 2008 at region where a known limit of 80 was reduced to 60 and the sign was hidden by a truck just passing and I was catched with 85. Not a problem with the knowing 80 by measuring telorance, but now it was.
The fine endet in an endlistment by limitation period. May, the problem here was the position of the radar. Reason of the new limit was bridge damages, but the radar was set far behind the bridge in an accelaration zone to the other highway. The treating office may got to much protest need to be trated then by judgment.