Compared to horizontal top-of-page menus, vertical left-side navigation has been found to yield faster navigation and greater user satisfaction. This may be simply due to vertical navigation bars being so common that users are used to them, but it may also be due to it being easier to scan down a menu than across. Interestingly, it appears you can combine a left-side vertical main menu and at-the-top horizontal submenus and get acceptable performance for multi-level menus. This is an attractive option if you can’t use flyouts and don’t want to squish content to the right to make room for a large wide vertical menu.
In contrast, another study found that users looked at horizontal top menu bars more than vertical left menu bars, leading the author to conclude that horizontal menus “performed best.” However, this may be an illustration of the dangers in interpreting eye-tracking data: just because users look toward something more, doesn’t mean it’s good. It could mean it’s hard to read. It could mean it’s a distraction. In this case, users’ primary task was apparently to consume the content on the page, rather than find content that’s somewhere else. If that’s true, you don’t want people looking at the navigation bar. High gaze time there means it’s distracting the users from their task.
This implies that one additional advantage of vertical menus is that they’re easy to ignore when users are not using them in addition to being easy to find and use when they do.