A surge in technology-fueled travel is pushing European governments to consider the effects of “overtourism" and how to curb it. According to the U.N. World Tourism Organization, international trips rose 6% in the first half of the year, aided by such innovations as the rise of low-cost airlines; bigger cruise ships; internet sites, including online booking, local reviews, smartphone mapping, and ride-hailing and home-sharing; and social media and its emphasis on personal-brand building through photos.