Traffic exchange is a popular concept on the Internet, referring to the exchange of web traffic on the World Wide Web. Usually, a central exchange site receives submissions from website operators. They sign up for traffic exchange networks. These people then browse other member sites on the exchange program to earn credits, which enable their sites to be viewed by other members. This increases the number of visitors to all the sites involved, but does so in a way that seems to have little benefit for the participants. Exchange programs typically enforce a 2-to-1 or 4-to-1 credit ratio, meaning members earn 0.5 or 0.25 in credit for visiting one member site, and each credit is translated to one page view for them. In theory, website owners would visit other sites through the central exchange program and thus channel more traffic back to their own sites. As the viewers are all website owners or operators, it is possible that some of them might find certain member sites interesting and thus make note of them on their own sites, sending more traffic their way. Most traffic programs also impose a time limit when members are browsing, ranging from 60 seconds to 10 seconds. Some incorporate the use of captcha to ensure user interaction, although there are exchange programs that let members browse without manually clicking, automatically moving on to the next site in rotation once the time limit is up. Almost all traffic exchange programs are free, although many of them offer special features to paid members and offer credits for purchase. Almost all traffic exchange programs encourage users to build their own referral networks, which would in turn help the referrers accumulate more credits. For example, when a referred member receives credits through browsing, the referrer would get a small portion of the credits earned.