Well, Max, either you can put your faith in the polling of Gallup or you can use common sense and try to take a reasonable look at the numbers.
First, is it really believeable that 45%+ of the population heads out to Church on Sunday, most between 8a and 12 noon? This would put more people and cars on the road in four hours than during a normal rush-hour of commuters on a weekday morning or evening (say 6a to 10a or 3p to 7p). Do you notice enormously congested roads on Sunday morning? No! Do you really believe that almost half the country is going to Church then? I don't.
Second, the two largest Churches in America happen to make headcounts of people every year - Catholics and Southern Baptists. While claiming 80 million members between them, they have combined attendance of around 25 million (about 5 million Southern Baptists and 20 million Catholics). Other large Churches that make headcounts (Episcopalians, Lutherans, Eastern Orthodox, Jews, etc.) report similar attendance figures of less than 1/3 of registered members counted BY HEAD in attendance, so we can safely say that of these 95 million members, 30 million are attending.
290 million Americans would mean 130 million Church attendees at 45%. Taking away groups enumerated above leaves 195 million Americans with a presumed 100 million attending. Given that about 1/3 of the population is not affiliated with any organized religion in any way (that's another 95 million), we are left having to believe that 100 million Protestants not yet counted above (Methodists, Evangelicals, Presbyterians, Northern Baptists, Pentecostal, etc.) attend Church at a rate of 100%. You are welcome to believe such poppycock. I don't.
It is quite likely, as comparisons of headcounts to surveys have repeatedly shown, that in the matter of Church attendance, people vastly overinflate their "good deeds" just like they exxagerate their tithing and other matters of generosity to pollsters.
I can tell you that in Philadelphia we have about 350,000 weekly Mass goers out of about 1.2 million registered in the ordinary parishes (I believe these figures exclude the many ethnic parishes here) - that's 30% attendance, and we are one of the better dioceses. Rates in Boston are known to be even lower, for example. If the 45% attendance rate of Catholics is not believable, I certainly don't believe a 45% rate for Protestants.
Much more likely, given the paucity of attendees at most Protestant Churches, is about 30% Catholic attendance, and 20-25% Protestant (130 million Protestants presumed population). This would work out to 20 million Catholics in Church and about 30 million Protestants (and Jews and Orthodox).
I am not able to speak to Protestant rates of attendance in the 1950's, although I recall from apologetic books that the rate may have been 30-35%. Catholic rates then were approximately double what they are today - around 60% by the diocesean headcounts. For example, San Francisco reported 200,000 attendees then and 100,000 today.
I won't dispute that Vatican II has accelerated a trend that was already evident in society since the turn of the century and certainly since WWII of declining Church attendance across the board; and at least in America and Europe, contributed (though I wouldn't say "caused") to making Catholics behave much more like Protestants.
This is a valid point, and I already mentioned it in a post to Salvation above. I assume that the people who answer "Yes" are ones who often go to church, or think of themselves as churchgoers, although not all of them actually attended church service last Sunday like they told the pollster. But the relevant question is, "Are there more liars among Catholics than protestants? Or did Catholics just lie more back in 1960?" I don't think either of those suppositions is the case, so the overall trend is clearly indicated by the graph even if the specific numbers must be taken with a grain of salt.
Do you notice enormously congested roads on Sunday morning? No!
I have often commented that Sunday morning roads are more congested where we live than any other time of the week. This is a fact. But in general, I would think that people don't travel far for services, so the vehicle miles on Sunday would be a lot lower. Not that all those cars are headed to Sunday service. Some may be headed to the mall. But as I found out to my shame soon after I moved here, the malls don't open until noon on Sunday here.
I won't dispute that Vatican II has accelerated a trend that was already evident in society since the turn of the century and certainly since WWII of declining Church attendance across the board;
I think the reality is that we had record attendance in the period between WWII and Vatican II (notice an ominous similarity in the numbering?). The percentages over the first few years of the study are flat at 74%, and there's no way that you're ever going to do much better than that. At the beginning of the study we were at a theoretical maximum, and probably doing much better than 50 or even 100 years earlier. Lots of immigrants from Catholic countries were anti-clerical, and there's always going to be a certain amount of fall out from divorce, etc.