Joe -
You stated a couple of times that there are still too many unanswered questions after an online automotive shopper has done their research and price discovery. I'm curious as to what information you feel a shopper can find on the dealer's website that they would not find in the online listings of a dealer doing a good job merchandising their inventory on ATC or cars.com.
There is so much specific vehicle information, multiple photos, dealership information, payment calculators, links to advice on buying, financing, leasing, insuring, online credit application, etc., what else is left for the shopper to ask other than, "do you still have the car?" And, in the case of a new car, if the dealer doesn't have THAT one, they'll have or can get another one just like it.
A prominent link to the dealer's web site (like ATC's BOB) is a great idea, but I don't believe the lack of such a link inhibits an online shopper from being able to proceed to the transaction step.
Studies have shown that what actually happens when the in-market online car buyer "vanishes" at the transaction step, as you put it, is that nearly 3/4 of them have done all of their online research and are now on their way to the dealership(s) that has the vehicle(s) of their choice. You can't touch, see or "drive" the product before the transaction step of an online air travel purchase, but you can with an automobile and that's the difference.
If I wanted to buy a new 55" flat screen television, I'd go online, do my research, do my price discovery and then.....I'd drive down to Best Buy or whatever retailer carried the TV I wanted so that I could first experience the picture and sound quality - and also make sure there wasn't something else I liked better - before making the transaction. Why would I call or email the store before doing so? I wouldn't, and studies show that most car buyers don't either.
As Alex pointed out, the problem is that most Dealer Principals don't incentivise or make it a condition of employment that salespeople and sales managers ACCURATELY ascertain from walk-in traffic what primarily influenced them to be in that dealership on that day to look at that car. Selecting "Drive by" should require management confirmation to be allowed in the stats.