White on green town signs (and older ones, such as black on white and inner-city blue on white) are only for informational purposes, not for speed limit purposes.
The town signs with black letters on white background used to introduce a speed limit in the past, like the signs with the town shape do now. But they doesn't mean that any more. So if someone tells you such a thing, remember about this. These signs shouldn't actually exist any more, they are no more in the highway code, they should have been exchanged to the green ones and the ones with the town shape, but they still exist (and it even sometimes happens that they install new ones) in some village roads. They have no meaning now, and even their informational function is played by them just on their own, it's not based on any regulations now.
Some time ago - I think it was in the 1990s, or shortly after 2000 - they replaced the white signs with a town name (which introduced a speed limit then too) with two separate signs: green signs with a town name (only informational purpose, no speed limit), and white signs with a town shape (speed limit to 50 km/h, or 60 km/h at night after 23, unless it is limited more, or the limit is weakened, by ordinary speed limit signs). There was a "switch-over" time, when both systems were in force, but it's for a long time that the white signs with a town name do not mean anything.
The white signs with a name in blue are equivalent to the green signs with a name in white, but they refer to city/town parts (like districts). If you meet them anywhere. They are not really common, not many cities/towns use them at all. They, of course, also don't introduce any speed limit.
Most people drive very slowly on the 4km stretch between Zabłudów and Żwierki on road 19. It's a pity they didn't put it where it might have helped.
There are some cases of municipalities in Poland, which established a local police (straż miejska - a kind of police independent of the main state-controlled one, which can be established by any municipality if it wants it) only for the purpose of erecting a speed camera and charging drivers on a road passing through a village for speeding... Just as a source of extra money to the municipality budget.
What took me forever to get used to was the idea of a junction "cancelling" the speed limit.
The idea behind that is, I think, that a driver entering the street from the side street at the intersection cannot know the speed limit that held before. So it simply cannot be so, that there is a junction and the speed limit still holds behind it without any signs. I have no idea, how they are dealing with this problem in the western countries, where an intersection doesn't cancel a speed limit.
There are broad disscusions in Poland how the relative road light sign should looks like to be more understandable for foreigners (and local ignorants)
It's an old post, but I will answer... It is so in some European countries and it is a very good idea. The basic meaning (not looking at the Polish highway code) of a green arrow light is the absolute priority in this direction. And it works so when you have three lights with arrows. But the conditional turn-right signal in Poland looks exactly the same (OK, the shape of the arrow is different, and it is a single signal cell instead of three ones, but it shouldn't be making any difference, while it is) and means something very different.
Not a long time ago - for sure at the end of the 1990s - it was so, that the conditional turn-right was allowed when there was a small sign (not a signal, an ordinary sign) with a green arrow on, if I remember well (I cannot find any photos on the Internet), black background below the main traffic light signals. Similarly like it is in Germany, but the sign was small and rectangular, not square, it was wide as the traffic light, but quite low, and it was installed below the general traffic light, not next to it. But then someone of the politicians found out that this is incompatible with some European norms (which is a total bullsh*t, see that Germans still have it and it works there). So first they removed these arrows and banned turning right on red, and then introduced this conditional signal with a green arrow instead. Without considering that they made it now really incompatible with the standards, according to which green arrow means absolute priority for the right-turn.
An issue important to remember here is that even with this conditional right-turn signal, you are obliged to stop due to the general red light, and only then you can start moving, giving priority to the pedestrians and to the traffic across. More or less like with the STOP sign. Interestingly, almost noone obeys this regulation, except for the driving learners' cars and the driving license exam ones (if you forget about that, you fail the exam), but it is good to know that.