A signal can be more a less reliable, miners will analyze their environment like every intelligent human beings are able to do so.
For example :
- transactions are slow because blocksize limit is often hit + nothing suspicous in the nodes activity + a majority of nodes signals their wish to increase the blocksize => miners will choose to increase the blocksize according to the nodes
- blocks are way below the limit + suspicious nodes activity + a majority of nodes signal their wish to increase the blocksize => miners will choose to no increase the blocksize
Basically it's likely that the blocksize will only be increased when it's needed.
As I understand it, miners would presumably communicate with each other, since other forms of ostracization of uncooperative fellows are entirely within their realm of possibility.
From the somewhat unified appearance of the Chinese miners at the last conference, a conservative approach seems to strongly match their behaviour.
And they would want node operators, businesses and end users supporting their move, so they would indicate a serious desire to raise the limit to the other stakeholders in advance and in plain language so that preparations could be made.
This is what I actually expect to happen, whether BU receives wider adoption or not - if there is no Economic Change Event beforehand. Or perhaps there has to be one for all to learn a lesson.