Comparing Store Visit Data
I recently increased the bid modifiers for campaigns which have a higher than average in store visit rate by 10% for 5 miles around all stores.
Re: Comparing Store Visit Data
Unfortunately, this is something you'll have to use your OWN data on. I totally feel your pain here but the best bet is to compare it using AdWords or GA data from a previous similar or identical period.
You might also want to try AdWords Campaign Experiments for this so that you can use a subset of your traffic to measure the impact of the change and that way, you eliminate the need to even consider seasonal variation.