From the Golden Gate Transit Website
Gas Prices and Bus Ridership
The correlation between gas prices and bus ridership is evident when comparing the change in ridership from this Fiscal Year 2008 to Fiscal Year 2007 (Column D) and the change in gas prices from the current month to previous month (Column G). Looking at those two columns, when gas prices go down, bus ridership goes down and when gas prices go up, bus ridership goes up (with exceptions: February, September and October 2008 when gas prices decreased and ridership increased).
Click the link to see the data.
The data is a little tricky to mine. The best comparison for ridership data is against the previous year at the same period in order to remove seasonal effects. But the data comparison they chose shows situations where "Gas prices decreased but ridership increased". They don't draw any conclusions, but since I am so smart I will.
Once gas prices increase above some pain threshold, a decrease in gas prices can still leave the gas price above the pain threshold. Of course, that doesn't explain why the riders weren't there the month before when the price first went over the pain threshold. I believe the answer is simply that for those who require a decent amount of pain in their wallet to switch from driving to transit - it takes a little bit of time and research to figure out the alternative solution.
The Sept 2008 data shows not only a 10% year on year gain, but also a 4.5% month to month gain, while in Sept 2007 there was a month to month net decrease. Sept 2008 was a good month for attracting new transit! This is despite the fact that gas prices were dropping! I posit that this is because prices were still almost four dollars, and by now the population had been dealing with such prices for four months or so, and the pain was starting to hit home. GG Transit attracted all sorts of new riders who finally got off their butts and figured out how this "bus thing" worked. I can confirm this anecdotally, I was occasionally taking the bus from Santa Rosa to SF in this time frame and the buses were packed, the park and ride lots were overflowing, and the riders waiting for the bus talked nothing but gas prices.
Notice that in November 2008 gas prices - and ridership - dropped dramatically. The column they hightlight shows it 5% less than Nov 2007. Again, I think it's more important to look at it differently. From October 2008 to Nov 2008, ridership dropped 17 percent. This could of course be seasonal, which is why they look at data compared with the same month in a prior year. But we can get some really useful data if we compare the deltas - the Oct-Nov delta for 2007 to the Oct-Nov delta for 2008. In 2007, this drop was 7.4%, but it was 17% in 2008.
There are a couple of potential factors here. First, Gas prices dropped with the economy, less people working, less people commuting in any form. Secondly, while there is a barrier to switching from driving to transit that slows the rise of transit usage when prices go up, this barrier does not exist when switching back to driving from transit (unless you sold you car I guess). Gas prices dropped, the bus started to seem less attractive.
The most interesting conclusion to try to draw is if - factoring out gas price increases and economic contraction - transit ridership is increasing in general. After the prices dropped and it all washed out - will public transit keep some of the riders that it attracted when it was more compelling from a financial standpoint?
Time will tell...