Originally Posted by Trakhak

Narrow bars: that's the direction the World Tour racers are going in. In fact, the UCI has stepped in with new rules concerning how narrow a bar is allowed.

From this page:

After a few ambitious riders had begun experimenting with extremely narrow handlebars in recent years, the UCI has stepped in to put a stop to the fun – or bring in some much needed safety limits (depending on your perspective).

While there was previously only a maximum permitted overall handlebar width (which still stands at 500mm for road, track and time trial bikes), a minimum overall handlebar width of 350mm has now been implemented for certain events as well.