Well of course you knew the routes to take, that is a drawback of the system. Scotus ruled that roadblocks are ok provided they are done out in the open by accepted standards. one of those is to notify the public of there locations prior to holding them.
Think how effective they would be if that was not the case.
I would argue that the 'probable cause' is found in the numbers of people that die and are injured day after day year after year.
Just like we have to act to stop the terrorism we have to take action to stop the dying on our nations roads.
They don't have to notify the public ahead of time in Canada, although they do usually publicize around holidays and such that there will be checkpoints, but they don't say where. It's just that naturally they tended to set them up on the higher traffic routes.
What you call probable cause does not meet the legal definition. For most of your country's history they would have been considered unconstitutional, just like asset forfeiture. The fact that some judges have stretched the meaning of the constitution to allow them doesn't make them right, does it? But I'll leave that argument to your countrymen. In this country they are perfectly constitutional and legal, but I still think they are a poor use of police resources, in most cases. Sometimes, for example on the main route out of an event, like the stock car races, where there's good reason to believe many intoxicated people might be hitting the road, I think they're useful.