As an automotive web provider, for us this is the million-dollar question. Obviously, there is no way of knowing for sure how traffic will be affected, if at all, and until the new feature has been rolled out for a significant amount of time, we believe organic traffic (through search engines, specifically Google) may take a slight hit eventually, mainly because the organic results on Google will be pushed lower down the page. As pictures of the cars, the dealership location and costs could all be displayed directly under the Google search box, right at the top.
On the other hand, website traffic may increase through other channels – specifically referral and direct, due to the boost in stock visibility. If a customer is interested in one your cars that they see on Google they are likely to click a link through to your website to gather further information or go to your website directly. If anything, the websites that are likely to suffer in regard to traffic are aggregator sites.