ROCHESTER, N.Y. -- Few vehicles are as effective at reaching large segments of the population as television, a fact that has established it as the favored medium for advertisers in many product categories.

For as long as that has been the case, however, TV networks and advertisers have been fearful of emerging competitors and technologies that threaten their route into consumers' minds. From the remote control to the Digital Video Recorder (DVR), there have long been predictions that live TV and its embedded advertisements were going to be adversely affected by consumers' ability to bypass commercials.

More recently, a different kind of threat has emerged from YouTube, the Internet's response to one-stop digital video viewing.

Recent research by Harris Interactive suggests that this fear may indeed be warranted. More than four in 10 (42 percent) online U.S. adults say they have watched a video at YouTube, and 14 percent say they visit the site frequently. Almost one in three (32 percent) of these frequent YouTube users say they are watching less TV as a result of the time they spend there.

However, YouTube has its own set of challenges as it tries to monetize the viewer traffic it has amassed. If YouTube is considering airing ads before its videos, they may be advised to halt that thinking; 73 percent of frequent YouTube users say they would visit the site less if it started including short video ads before every clip.

Of all frequent YouTube users, two-thirds (66 percent) claim they are sacrificing other activities when on YouTube. Although their visits to the site are most likely to have been at the expense of visiting other websites (36 percent), time spent watching TV is next most likely to have taken a hit (32 percent).

YouTube also cuts into e-mail and other online social networking (20 percent), work/homework (19 percent), playing video games (15 percent), watching DVD(s) (12 percent) and even spending time with friends and family in person (12 percent).

Further compounding the problem for the TV and advertising, YouTube usage is greatest among the group already hardest to reach through television advertising: young males. Over three-quarters (76 percent) of 18- to 24-year-old males say they have watched a video at YouTube, and 41 percent visit YouTube frequently.

However, YouTube faces challenges of its own as it tries to cash in on the house that it has built. When asked if the inclusion of short commercials before every clip would change how often they will visit YouTube, nearly three-quarters of adults who frequently visit the site say they would visit it a lot (31 percent) or a little (42 percent) less often as a result.

Indeed, in the last year, TV networks have successfully experimented with airing of TV episodes with commercials on their websites. Nearly as many online adults (41 percent) say they have watched a video at a TV network website as they have at YouTube (42 percent). It seems like TV networks can get away with advertising more easily.